fix(bufferTime): correct bufferCreationInterval check for null

- closes ReactiveX#1944[

src/operator/bufferTime.ts

@@ -120,7 +120,7 @@ class BufferTimeSubscriber<T> extends Subscriber<T> {
               private scheduler: Scheduler) {
     super(destination);
     const context = this.openContext();
-    this.timespanOnly = bufferCreationInterval == null || bufferCreationInterval < 0;
+    this.timespanOnly = bufferCreationInterval === null || bufferCreationInterval < 0;
     if (this.timespanOnly) {
       const timeSpanOnlyState = { subscriber: this, context, bufferTimeSpan };
       this.add(context.closeAction = scheduler.schedule(dispatchBufferTimeSpanOnly, bufferTimeSpan, timeSpanOnlyState));