libavformat/rtsp.c: Reply to GET_PARAMETER requests

Some encoders send GET_PARAMETER requests as a keep-alive mechanism.
If the client doesn't reply with an OK message, the encoder will close
the session.  This was encountered with the impath i5110 encoder, when
the RTSP Keep-Alive checkbox is enabled under streaming settings.
Alternatively one may set the X-No-Keepalive: 1 header, but this is more
of a workaround.  It's better practice to respond to an encoder's
keep-alive request, than disable the mechanism which may be manufacturer
specific.

Signed-off-by: Hayden Myers <hmyers@skylinenet.net>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtsp.c

@@ -1259,7 +1259,8 @@ start:
         char base64buf[AV_BASE64_SIZE(sizeof(buf))];
         const char* ptr = buf;
 
-        if (!strcmp(reply->reason, "OPTIONS")) {
+        if (!strcmp(reply->reason, "OPTIONS") ||
+            !strcmp(reply->reason, "GET_PARAMETER")) {
             snprintf(buf, sizeof(buf), "RTSP/1.0 200 OK\r\n");
             if (reply->seq)
                 av_strlcatf(buf, sizeof(buf), "CSeq: %d\r\n", reply->seq);