fix(mergeMapTo): mergeMapTo result should complete

Fix mergeMapTo operator to properly complete when all inner Observables
have completed and so has the outer completed. Fix the `active` count
in mergeMapTo to increment when a new inner is opened, and to decrement
when an existing inner is closed.

src/operators/mergeMapTo-support.ts

@@ -41,7 +41,7 @@ export class MergeMapToSubscriber<T, R, R2> extends OuterSubscriber<T, R> {
       if (ish === errorObject) {
         destination.error(ish.e);
       } else {
-        this.active--;
+        this.active++;
         this._innerSub(ish, destination, resultSelector, value, index);
       }
     } else {