rtspsrc: 551 should not result in an unhandled error

When an RTSP camera returns 551 (method not supported), disable the method and continue. It should not throw an 'unhandled error'.

This avoids applications picking up errors on simple RTSP implementations (when a camera does not support a PAUSE, some HikVision cameras).

+      /* Some cameras (e.g. HikVision DS-2CD2732F-IS) return "551
+       * Option not supported" when a command is sent that is not implemented
+       * (e.g. PAUSE). Instead; it should return "501 Not Implemented".
+       *
+       * This is wrong, as previously, the camera did announce support
+       * for PAUSE in the OPTIONS.
+       *
+       * In this case, handle the 551 as if it was 501 to avoid throwing
+       * errors to application level. */