Subject::lift has incorrect type signature

**RxJS version:** 5.3.0+

**Additional information:**

`lift` is defined on `Observable<T>` as:
```typescript
lift<R>(operator: Operator<T, R>): Observable<R>;
```

`Subject<T>` defines `lift` with the signature:
```typescript
lift<R>(operator: Operator<T, R>): Observable<T>
```
The return type should probably be `Observable<R>` rather than `Observable<T>`