Support for passing objects by ref to OnNext

I have recently been using Rx for a simple event bus to publish/subscribe to events throughout my application, the system needed to allow events to be marked as handled and in that case, they wouldn't propagate any further. I realised that with Rx, everything is a copy so there is no way to update the source event.

I created a `RefSubject` and `IRefObserver/IRefObservable` using `public delegate void RefAction(ref T item)` and passing everything by `ref` which works nicely. The problem is that I would have to implement every operator manually, not so bad right now as I'm only using `Where` but it would be nice if passing by `ref` to `OnNext` was officially supported.

Is there a major reason that it's not in the library right now? Thanks