fix(Subject): lift signature is now appropriate for stricter TypeScript 2.4 checks (#2722)

Type safety wasn't really gauranteed with lift before because we survived for a long time with an incorrect type signature, TypeScript 2.4 introduces stricter type-checking, and all of a sudden `lift` on `Subject` was breaking builds for those that are riding the wave of the latest-and-greatest.

Author: benlesh

src/Subject.ts

@@ -43,7 +43,7 @@ export class Subject<T> extends Observable<T> implements ISubscription {
     return new AnonymousSubject<T>(destination, source);
   }
 
-  lift<R>(operator: Operator<T, R>): Observable<T> {
+  lift<R>(operator: Operator<T, R>): Observable<R> {
     const subject = new AnonymousSubject(this, this);
     subject.operator = <any>operator;
     return <any>subject;