fix(groupBy): fix bugs related to group resets

When using durationSelector on groupBy operator, completed group
Observables were not being properly removed from the underlying Map of
groups by key. This fixes the following test cases:
- Observable.prototype.groupBy() should allow using a keySelector,
elementSelector, and durationSelector
- Observable.prototype.groupBy() should allow using a keySelector and a
durationSelector, outer throws
- Observable.prototype.groupBy() should allow using a durationSelector,
but keySelector throws
- Observable.prototype.groupBy() should allow using a durationSelector,
but elementSelector throws
- Observable.prototype.groupBy() should allow an inner to be unsubscribed
early but other inners continue, with durationSelector
- Observable.prototype.groupBy() should allow inners to be unsubscribed
early at different times, with durationSelector
- Observable.prototype.groupBy() should return inners that when subscribed
late exhibit hot behavior

src/operators/groupBy.ts

@@ -116,7 +116,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
   }
 
   removeGroup(key: string) {
-    this.groups[key] = null;
+    this.groups.delete(key);
   }
 }
 

src/util/FastMap.ts

@@ -19,7 +19,7 @@ export default class FastMap {
   forEach(cb, thisArg) {
     const values = this._values;
     for (let key in values) {
-      if (values.hasOwnProperty(key)) {
+      if (values.hasOwnProperty(key) && values[key] !== null) {
         cb.call(thisArg, values[key], key);
       }
     }