Fix MissingMethodException that mysteriously is thrown when the type isn't specified

In my last PR, I used the Observable.FromEventPattern method to listen to the Disposed event.
Somehow, in my application where I use ReactiveSockets, the listener suddenly threw a MissingMethodException but it works fine in ReactiveSocket's unit tests.

Adding the generic types fixes this problem.

Why this solves the problem, or why there was a problem in first place is beyond my comprehension...