message: test(mergeAll): add test with multiple types
diff: import { of } from 'rxjs';
  import { mergeAll } from 'rxjs/operators';
  +import { a$, b$ } from '../helpers';
  -it('should infer correctly', () => {
  - const o = of(of(1, 2, 3)).pipe(mergeAll()); // $ExpectType Observable<number>
  +it('should infer correctly with sources of same type', () => {
  + const o = of(a$, a$).pipe(mergeAll()); // $ExpectType Observable<A>
  +});
  +
  +it('should infer correctly with sources of different types', () => {
  + const o = of(a$, b$).pipe(mergeAll()); // $ExpectType Observable<A | B>
  });
  it('should enforce types', () => {
  - const o = of(1, 2, 3).pipe(mergeAll()); // $ExpectError
  + const o = a$.pipe(mergeAll()); // $ExpectError
  });
  