Flush() being called for ResponseWriter which doesn't implement the interface

[mangas]

Versions of relevant software used
v0.9.6

What happened
Panic when cast to interface

What you expected to happen
Method should only be called upon successful cast

How to reproduce it (as minimally and precisely as possible):
Call webGrpcResponse.Flush() with wrapped being an instance of http.ResponseWriter

[johanbrandhorst]

I left a comment on #479 already, but basically; why do you need this? I don't think it's as clear cut as just doing it opportunistically. Is there any option for you to implement Flush on your ResponseWriter instead?

[mangas]

There are 2 reason why I think this should be solved in the wrapper.

1. The api semantics don't specify anything regarding the ResponseWriter having to implement http.Flusherand should in that case treat as an optional feature or change the api to take a Flusher instance.

2. Implementing a "dummy" Flush func also means flush might be called assuming the normal semantics and in fact nothing will happen