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

lib/http.c

@@ -3402,7 +3402,8 @@ CURLcode Curl_http_readwrite_headers(struct SessionHandle *data,
 
     }
     else if(checkprefix("Content-Encoding:", k->p) &&
-            data->set.str[STRING_ENCODING]) {
+            (data->set.str[STRING_ENCODING] ||
+             conn->httpversion == 20)) {
       /*
        * Process Content-Encoding. Look for the values: identity,
        * gzip, deflate, compress, x-gzip and x-compress. x-gzip and