fix(subscribe): allow partial listeners (in TypeScript)

we should allow subscribe to accept listener objects that are missing some fields, just like
addListener accepts

src/index.ts

@@ -93,10 +93,10 @@ function internalizeProducer<T>(producer: Producer<T> & Partial<InternalProducer
 }
 
 class StreamSub<T> implements Subscription {
-  constructor(private _stream: Stream<T>, private _listener: Listener<T>) {}
+  constructor(private _stream: Stream<T>, private _listener: InternalListener<T>) {}
 
   unsubscribe(): void {
-    this._stream.removeListener(this._listener);
+    this._stream._remove(this._listener);
   }
 }
 
@@ -1225,9 +1225,9 @@ export class Stream<T> implements InternalListener<T> {
    * @param {Listener} listener
    * @returns {Subscription}
    */
-  subscribe(listener: Listener<T>): Subscription {
+  subscribe(listener: Partial<Listener<T>>): Subscription {
     this.addListener(listener);
-    return new StreamSub<T>(this, listener);
+    return new StreamSub<T>(this, listener as InternalListener<T>);
   }
 
   /**

tests/stream.ts

@@ -413,5 +413,20 @@ describe('Stream', () => {
 
       done();
     });
+
+    it('should accept a partial listener', (done: any) => {
+      const stream = xs.empty();
+      const noop = (): void => void 0;
+      const listener = {
+        next: noop,
+      };
+
+      const subscription = stream.subscribe(listener);
+
+      assert.equal(typeof subscription, 'object');
+      assert.equal(typeof subscription.unsubscribe, 'function');
+
+      done();
+    });
   });
 });