fix(Subject): lift signature is now appropriate for stricter TypeScri…

…pt 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.

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;