fix(Observable): align type definition of subscriber with Observable

- closes ReactiveX#2166

spec/operators/groupBy-spec.ts

@@ -1378,7 +1378,7 @@ describe('Observable.prototype.groupBy', () => {
       observer.complete();
     }).groupBy(
       (x: number) => x % 2,
-      (x: string) => x + '!'
+      (x: number) => x + '!'
     );
 
     expect(result instanceof MyCustomObservable).to.be.true;

src/Observable.ts

@@ -37,7 +37,7 @@ export class Observable<T> implements Subscribable<T> {
    * can be `next`ed, or an `error` method can be called to raise an error, or
    * `complete` can be called to notify of a successful completion.
    */
-  constructor(subscribe?: <R>(this: Observable<T>, subscriber: Subscriber<R>) => TeardownLogic) {
+  constructor(subscribe?: (this: Observable<T>, subscriber: Subscriber<T>) => TeardownLogic) {
     if (subscribe) {
       this._subscribe = subscribe;
     }
@@ -53,7 +53,7 @@ export class Observable<T> implements Subscribable<T> {
    * @param {Function} subscribe? the subscriber function to be passed to the Observable constructor
    * @return {Observable} a new cold observable
    */
-  static create: Function = <T>(subscribe?: <R>(subscriber: Subscriber<R>) => TeardownLogic) => {
+  static create: Function = <T>(subscribe?: (subscriber: Subscriber<T>) => TeardownLogic) => {
     return new Observable<T>(subscribe);
   };