fix(throttleTime): fix double emission with leading and trailing enabled

Fix an issue with throttleTime emitting both leading and trailing values in the same time window.

Double emission problem:
source:   a123b12-c-23d-2-ef---
expected: a---b---c---d---e---f
actual:   a---b1---c2---2-e---f

Closes ReactiveX#2466 and ReactiveX#2727.
Follows ReactiveX#2749 and ReactiveX#2864.

BREAKING CHANGE: throttleTime no longer emits both leading and trailing
values in the same time window.

src/internal/operators/throttleTime.ts

@@ -130,7 +130,7 @@ class ThrottleTimeSubscriber<T> extends Subscriber<T> {
         this._hasTrailingValue = true;
       }
     } else {
-      this.add(this.throttled = this.scheduler.schedule<DispatchArg<T>>(dispatchNext, this.duration, { subscriber: this }));
+      this.throttle();
       if (this.leading) {
         this.destination.next(value);
       } else if (this.trailing) {
@@ -151,17 +151,27 @@ class ThrottleTimeSubscriber<T> extends Subscriber<T> {
 
   clearThrottle() {
     const throttled = this.throttled;
+
     if (throttled) {
+      throttled.unsubscribe();
+      this.remove(throttled);
+      this.throttled = null;
+
       if (this.trailing && this._hasTrailingValue) {
         this.destination.next(this._trailingValue);
         this._trailingValue = null;
         this._hasTrailingValue = false;
+
+        if (this.leading && this.trailing) {
+          this.throttle();
+        }
       }
-      throttled.unsubscribe();
-      this.remove(throttled);
-      this.throttled = null;
     }
   }
+
+  throttle() {
+    this.add(this.throttled = this.scheduler.schedule<DispatchArg<T>>(dispatchNext, this.duration, { subscriber: this }));
+  }
 }
 
 interface DispatchArg<T> {