fix(forkJoin): test for object literal (#4741)

* test(forkJoin): add failing test for #4737

* fix(forkJoin): test for object literal not observable

Closes #4737.

spec/observables/forkJoin-spec.ts

@@ -504,5 +504,18 @@ describe('forkJoin', () => {
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
       expectSubscriptions(e2.subscriptions).toBe(e2subs);
     });
+
+    it('should accept promise as the first arg', done => {
+      const e1 = forkJoin(Promise.resolve(1));
+      const values: number[][] = [];
+
+      e1.subscribe({
+        next: x => values.push(x),
+        complete: () => {
+          expect(values).to.deep.equal([[1]]);
+          done();
+        }
+      });
+    });
   });
 });

src/internal/observable/forkJoin.ts

@@ -147,7 +147,7 @@ export function forkJoin(
       return forkJoinInternal(first, null);
     }
     // TODO(benlesh): isObservable check will not be necessary when deprecated path is removed.
-    if (isObject(first) && !isObservable(first)) {
+    if (isObject(first) && Object.getPrototypeOf(first) === Object.prototype) {
       const keys = Object.keys(first);
       return forkJoinInternal(keys.map(key => first[key]), keys);
     }