http2: force "drainage" of streams

... which is necessary since the socket won't be readable but there is
data waiting in the buffer.

lib/multi.c

@@ -1500,6 +1500,8 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         break;
       }
 
+      DEBUGF(infof(data, "%s:%d call Curl_readwrite\n", __func__, __LINE__));
+
       /* read/write data if it is ready to do so */
       result = Curl_readwrite(data->easy_conn, &done);