http2: rely on content-encoding header

A server might respond with a content-encoding header and a response
that was encoded accordingly in HTTP-draft-09/2.0 mode, even if the
client did not send an accept-encoding header earlier. The server might
not send a content-encoding header if the identity encoding was used to
encode the response.

See:
http://tools.ietf.org/html/draft-ietf-httpbis-http2-09#section-9.3