Getting response body in Starlette middleware no longer correct

[nwrahnema]

Minor issue but the types for the solution posted in this issue #874 are no longer correct after this change d771bb7 where _StreamingResponse now inherits from Response instead of StreamingResponse.

While that code does technically still work, in general I think it makes writing middleware more confusing as isinstance(response, StreamingResponse) will strangely not work as expected.

[nwrahnema]

#2801