wsgi response body support bytearray

I have some python extension generate responses as bytearray, because bytearray is more efficent in some cases, but i don't want to copy it into a string before return it as wsgi response, so i want to return bytearray directly.

wsgiref and other pure python wsgi gateways already support this somehow, i guess they just don't check the types, and `socket.send` works for both. So i hope uwsgi can also support this use case.

There's another improvement in this patch, because wsgi specification force wsgi application return an iterable, so simple applications usually return one-length list whose only element contains the whole response body, so i add a check for this situation and write the response out, which can save a round strips between the caller, in case of gevent, it saves a greenlet switch.