MessageBag Clear and ClearKey

Originally submitted at #4014 against master, should be okay for 4.1 though.

I may have been overlooking something, or maybe I just over engineered my application a bit, however I thought add a `clear` and `clearKey` method might be useful on the MessageBag.  I won't go into detail on my use case, but I was using a singleton instance and got into a bit of trouble.

Feel free to just shoot this down, I can just as easily add it in my extended class.