fix(throttleTime): emit single value with trailing enabled

[MatthiasKunnen]

throttleTime didn't emit single values when leading was disabled and trailing enabled.

e.g.:

source:   -a--------------------|
time:     ----|                  
previous: ----------------------|
current:  -----a----------------|

I've added tests to verify this behavior and added another commit to fix it.

Closes #2859 and #4491.

perhaps irrelevant
I have another branch ready to fix some other problems related to throttleTime which I will create a PR for when this one is merged.

[coveralls]

Pull Request Test Coverage Report for Build 8355

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 32 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.2%) to 96.652%

---

Files with Coverage Reduction	New Missed Lines	%
src/internal/scheduler/VirtualTimeScheduler.ts	1	83.52%
src/internal/operators/partition.ts	1	75.0%
src/internal/observable/combineLatest.ts	1	96.34%
src/internal/operators/throwIfEmpty.ts	1	93.94%
src/internal/observable/from.ts	1	84.09%
src/internal/OuterSubscriber.ts	1	50.0%
src/internal/observable/race.ts	1	95.0%
src/internal/Observable.ts	3	91.67%
src/internal/util/not.ts	4	20.0%
src/internal/observable/ConnectableObservable.ts	18	55.46%

Totals
Change from base Build 8066:	-0.2%
Covered Lines:	5254
Relevant Lines:	5436

---

💛 - Coveralls

[MatthiasKunnen]

@cartant, I'm notifying you because I am a bit desperate for a review and you commented on #4491 of which this PR is a replacement.

I'm hoping you don't mind the notification.

[cartant]

I've reviewed this, but I'd like to make some changes to the test descriptions - some of which are separate to the changes you've made in this PR. I'm unable to push make changes to your PR, so you can either ensure that this box is checked:

Or you can apply the following patch that includes the changes:

From 43005d331227b879591ee197704a7c5089ce0d54 Mon Sep 17 00:00:00 2001
From: Nicholas Jamieson <nicholas@cartant.com>
Date: Sat, 13 Apr 2019 15:11:47 +1000
Subject: [PATCH] chore: improve test descriptions

And remove an asDiagram call made with the same argument.
---
 spec/operators/throttleTime-spec.ts | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/spec/operators/throttleTime-spec.ts b/spec/operators/throttleTime-spec.ts
index bbaab515..4e65054a 100644
--- a/spec/operators/throttleTime-spec.ts
+++ b/spec/operators/throttleTime-spec.ts
@@ -141,10 +141,10 @@ describe('throttleTime operator', () => {
   });
 
   describe('throttleTime(fn, { leading: true, trailing: true })', () => {
-    asDiagram('throttleTime(fn, { leading: true, trailing: true })')('should immediately emit the first value in each time window', () =>  {
+    asDiagram('throttleTime(fn, { leading: true, trailing: true })')('should immediately emit the first and last values in each time window', () =>  {
       const e1 =   hot('-a-xy-----b--x--cxxx--|');
       const e1subs =   '^                     !';
-      const t =  time( '----|                 ');
+      const t =  time( '----|                  ');
       const expected = '-a---y----b---x-c---x-|';
 
       const result = e1.pipe(throttleTime(t, rxTestScheduler, { leading: true, trailing: true }));
@@ -153,7 +153,7 @@ describe('throttleTime operator', () => {
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
     });
 
-    it('should emit the first value if only a single one is given', () => {
+    it('should emit the value if only a single one is given', () => {
       const e1 =   hot('-a--------------------|');
       const t =   time('----|                  ');
       const expected = '-a--------------------|';
@@ -165,10 +165,10 @@ describe('throttleTime operator', () => {
   });
 
   describe('throttleTime(fn, { leading: false, trailing: true })', () => {
-    asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should immediately emit the first value in each time window', () =>  {
+    asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should immediately emit the last value in each time window', () =>  {
       const e1 =   hot('-a-xy-----b--x--cxxx--|');
       const e1subs =   '^                     !';
-      const t =  time( '----|                 ');
+      const t =  time( '----|                  ');
       const expected = '-----y--------x-----x-|';
 
       const result = e1.pipe(throttleTime(t, rxTestScheduler, { leading: false, trailing: true }));
@@ -177,10 +177,10 @@ describe('throttleTime operator', () => {
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
     });
 
-    asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should emit the last throttled value when complete', () => {
+    it('should emit the last throttled value when complete', () => {
       const e1 =   hot('-a-xy-----b--x--cxx|');
       const e1subs =   '^                  !';
-      const t =   time('----|              ');
+      const t =   time('----|               ');
       const expected = '-----y--------x----(x|)';
 
       const result = e1.pipe(throttleTime(t, rxTestScheduler, { leading: false, trailing: true }));
@@ -189,7 +189,7 @@ describe('throttleTime operator', () => {
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
     });
 
-    it('should emit the first value if only a single one is given', () => {
+    it('should emit the value if only a single one is given', () => {
       const e1 =   hot('-a--------------------|');
       const t =   time('----|                  ');
       const expected = '-----a----------------|';
-- 
2.17.1.windows.2

[MatthiasKunnen]

Thank you for the review, I've applied the patch.

[MatthiasKunnen]

Thank you for the review and merge @benlesh. I'll create another PR addressing other issues with throttleTime later today.

[MatthiasKunnen]

@benlesh and @cartant, the follow-up PR is #4727.