rtsp: Use GET_PARAMETER for keep-alive for generic RTSP servers

According to the RFC, GET_PARAMETER should be used for
this, and according to a report from Tim Ouellette,
OPTIONS doesn't work for keeping the connection alive for some
servers. Also, live555 uses GET_PARAMETER for this purpose.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/rtspdec.c

@@ -340,7 +340,7 @@ retry:
 
     /* send dummy request to keep TCP connection alive */
     if ((av_gettime() - rt->last_cmd_time) / 1000000 >= rt->timeout / 2) {
-        if (rt->server_type == RTSP_SERVER_WMS) {
+        if (rt->server_type != RTSP_SERVER_REAL) {
             ff_rtsp_send_cmd_async(s, "GET_PARAMETER", rt->control_uri, NULL);
         } else {
             ff_rtsp_send_cmd_async(s, "OPTIONS", "*", NULL);