fix(forkJoin): add type signature for single observable with selector

Add type signature for using forkJoin with single observable as first parameter
and selector function as second parameter, so that typings would not prevent
usage which is permitted and properly handled by operator.

Closes ReactiveX#2347

spec/observables/forkJoin-spec.ts

@@ -1,7 +1,12 @@
 import {expect} from 'chai';
 import * as Rx from '../../dist/cjs/Rx';
-declare const {hot, expectObservable, expectSubscriptions, type};
 import {lowerCaseO} from '../helpers/test-helper';
+import marbleTestingSignature = require('../helpers/marble-testing'); // tslint:disable-line:no-require-imports
+
+declare const {type};
+declare const hot: typeof marbleTestingSignature.hot;
+declare const expectObservable: typeof marbleTestingSignature.expectObservable;
+declare const expectSubscriptions: typeof marbleTestingSignature.expectSubscriptions;
 
 const Observable = Rx.Observable;
 

src/observable/ForkJoinObservable.ts

@@ -25,6 +25,7 @@ export class ForkJoinObservable<T> extends Observable<T> {
   static create<T, T2, T3, T4>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, v3: SubscribableOrPromise<T3>, v4: SubscribableOrPromise<T4>): Observable<[T, T2, T3, T4]>;
   static create<T, T2, T3, T4, T5>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, v3: SubscribableOrPromise<T3>, v4: SubscribableOrPromise<T4>, v5: SubscribableOrPromise<T5>): Observable<[T, T2, T3, T4, T5]>;
   static create<T, T2, T3, T4, T5, T6>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, v3: SubscribableOrPromise<T3>, v4: SubscribableOrPromise<T4>, v5: SubscribableOrPromise<T5>, v6: SubscribableOrPromise<T6>): Observable<[T, T2, T3, T4, T5, T6]>;
+  static create<T, R>(v1: SubscribableOrPromise<T>, project: (v1: T) => R): Observable<R>;
   static create<T, T2, R>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, project: (v1: T, v2: T2) => R): Observable<R>;
   static create<T, T2, T3, R>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, v3: SubscribableOrPromise<T3>, project: (v1: T, v2: T2, v3: T3) => R): Observable<R>;
   static create<T, T2, T3, T4, R>(v1: SubscribableOrPromise<T>, v2: SubscribableOrPromise<T2>, v3: SubscribableOrPromise<T3>, v4: SubscribableOrPromise<T4>, project: (v1: T, v2: T2, v3: T3, v4: T4) => R): Observable<R>;