fix(flatten): Fix flatten operator related to MemoryStream(s)

When increasing strictness, in particular "strictFunctionTypes",
flatten was quite broken even when trying a simple cyclejs example
`sources.HTTP.select().flatten()`

This commit fixes the compile errors and the return types.

Resolve #262.

src/index.ts

@@ -1764,8 +1764,8 @@ export class Stream<T> implements InternalListener<T> {
    *
    * @return {Stream}
    */
-  flatten<R>(this: Stream<Stream<R>>): T {
-    return new Stream<R>(new Flatten(this)) as T & Stream<R>;
+  flatten<R>(this: Stream<Stream<R>>): Stream<R> {
+    return new Stream<R>(new Flatten(this));
   }
 
   /**
@@ -1976,7 +1976,7 @@ export class Stream<T> implements InternalListener<T> {
 
 export class MemoryStream<T> extends Stream<T> {
   private _v?: T;
-  private _has: boolean = false;
+  private _has?: boolean = false;
   constructor(producer: InternalProducer<T>) {
     super(producer);
   }

tests/operator/flatten.ts

@@ -335,8 +335,11 @@ describe('Stream.prototype.flatten', () => {
     });
   });
 
-  it.skip('should allow to flatten with inner being a MemoryStream', function() {
-    // @TODO: actually make this next line compile
-    // xs.of(xs.of(1).remember()).flatten();
+  it('should allow to flatten while playing around with MemoryStream(s) and correct type signature', function() {
+    xs.of(xs.of(1)).flatten() // Stream<Stream<number>> => Stream<number>
+    xs.of(xs.of(1).remember()).flatten() // Stream<MemoryStream<number>> => Stream<number>
+    xs.of(xs.of(1)).remember().flatten() // MemoryStream<Stream<number>> => Stream<number>
+    // xs.of(xs.of(1).remember()).remember().flatten() // Doesn't compile
+    xs.of(xs.of(1).remember()).remember().flatten<number>() // MemoryStream<MemoryStream<number>> => Stream<number>
   })
 });