src: workaround moment issue with DST transitions

On DST end transition, setting the date to the `startOf` `hour`,
`minute` or `second` after having added 1 `hour`, `minute` or `second`
to the date, would result in the date set to the initial value causing
an infinite loop.

Example:

```js
'use strict';

const moment = require('moment-timezone');

var m = moment(new Date('Sun Oct 30 2016 02:00:00 GMT+0200'));
console.log(m.toString(),
            '<-- Just 1 hour before the DST end @ Europe/Madrid');
m.add(1, 'hours');
console.log(m.toString(),
            '<-- DST end transition @ Europe/Madrid works correctly');
m.startOf('hour');
console.log(m.toString(),
            '<-- Smthing wrong?? It goes back to the previous hour');
```

Fixes: harrisiirak#72
Ref: moment/moment#2749

lib/date.js

@@ -15,15 +15,27 @@ CronDate.prototype.addDay = function() {
 };
 
 CronDate.prototype.addHour = function() {
+  const prev = this.getTime();
   this._date.add(1, 'hour').startOf('hour');
+  if (this.getTime() <= prev) {
+    this._date.add(1, 'hour');
+  }
 };
 
 CronDate.prototype.addMinute = function() {
+  const prev = this.getTime();
   this._date.add(1, 'minute').startOf('minute');
+  if (this.getTime() < prev) {
+    this._date.add(1, 'hour');
+  }
 };
 
 CronDate.prototype.addSecond = function() {
+  const prev = this.getTime();
   this._date.add(1, 'second').startOf('second');
+  if (this.getTime() < prev) {
+    this._date.add(1, 'hour');
+  }
 };
 
 CronDate.prototype.getDate = function() {

test/timezone.js

@@ -264,6 +264,113 @@ test('It works on DST end', function(t) {
     t.throws(function() {
         date = interval.next();
     });
+
+    options = {
+        currentDate : new Date('Sun Oct 29 2016 01:00:00 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 29, '29th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 29 2016 02:59:00 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 29, '29th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 29 2016 02:59:59 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 29, '29th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 30 2016 01:00:00 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 30 2016 01:59:00 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 30 2016 01:59:59 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
+
+    options = {
+        currentDate : new Date('Sun Oct 30 2016 02:59:00 GMT+0200')
+    }
+
+    interval = CronExpression.parse('0 12 * * *', options);
+    t.ok(interval, 'Interval parsed');
+
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 30, '30th');
+    date = interval.next();
+    t.equal(date.getHours(), 12, '12');
+    t.equal(date.getDate(), 31, '31st');
    } catch (err) {
     t.ifError(err, 'Interval parse error');
   }