http2: enforce gzip auto-decompress

As this is mandated by the http2 spec draft-09

lib/http2.c

@@ -410,6 +410,7 @@ CURLcode Curl_http2_request_upgrade(Curl_send_buffer *req,
   free(base64);
 
   k->upgr101 = UPGR101_REQUESTED;
+  k->auto_decoding = GZIP;
 
   return result;
 }
@@ -640,6 +641,8 @@ int Curl_http2_switched(struct connectdata *conn)
   httpc->data = NULL;
   httpc->datalen = 0;
 
+  conn->data->req.auto_decoding = GZIP;
+
   /* Put place holder for status line */
   Curl_add_buffer(httpc->header_recvbuf, "HTTP/2.0 200\r\n", 14);