fix(Subject): lift signature is now appropriate for strictor TS 2.4 c…

…hecks

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.

BREAKING CHANGES: It is possible that the type signature change to `Subject.prototype.lift` will break someone's build, but it seems unlikely given that TS 2.3 and lower didn't seem to give the proper type-safety guarantees with that method. Because this particular issue is blocking people wanting to use TS 2.4, and because it is very unlikely to cause any issues to anyone, we are not going to increment a major version for this particular possibly breaking change. Apologies to anyone this breaks.

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;