Fix rmvb video can not play under DLNA

Or will report "Could not find handler for /videos/xxx/stream.rm" error
in server side.

Test OK with Kodi and gupnp-tools.

MediaBrowser.Api/Playback/Progressive/VideoService.cs

@@ -37,6 +37,7 @@ namespace MediaBrowser.Api.Playback.Progressive
     [Route("/Videos/{Id}/stream.mov", "GET")]
     [Route("/Videos/{Id}/stream.iso", "GET")]
     [Route("/Videos/{Id}/stream.flv", "GET")]
+    [Route("/Videos/{Id}/stream.rm", "GET")]
     [Route("/Videos/{Id}/stream", "GET")]
     [Route("/Videos/{Id}/stream.ts", "HEAD")]
     [Route("/Videos/{Id}/stream.webm", "HEAD")]