fix(Subject): make value parameter for next non-optional

Prevents emitting undefined on strictly-typed Subjects

BREAKING CHANGE:
If you are using TypeScript called next() without a value before, TypeScript will error. If the Subject is of type void, explicitely pass undefined to next().

closes ReactiveX#2852

src/Subject.ts

@@ -49,7 +49,7 @@ export class Subject<T> extends Observable<T> implements ISubscription {
     return <any>subject;
   }
 
-  next(value?: T) {
+  next(value: T) {
     if (this.closed) {
       throw new ObjectUnsubscribedError();
     }

src/operators/repeatWhen.ts

@@ -46,7 +46,7 @@ class RepeatWhenOperator<T> implements Operator<T, T> {
  */
 class RepeatWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
 
-  private notifications: Subject<any>;
+  private notifications: Subject<void>;
   private retries: Observable<any>;
   private retriesSubscription: Subscription;
   private sourceIsBeingSubscribedTo: boolean = true;
@@ -81,7 +81,7 @@ class RepeatWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
       }
 
       this._unsubscribeAndRecycle();
-      this.notifications.next();
+      this.notifications.next(undefined);
     }
   }