Fix subtitle encoder if webvtt is requested

MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs

@@ -293,7 +293,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
                 return true;
             }
 
-            if (string.Equals(format, SubtitleFormat.VTT, StringComparison.OrdinalIgnoreCase))
+            if (string.Equals(format, SubtitleFormat.VTT, StringComparison.OrdinalIgnoreCase) || string.Equals(format, SubtitleFormat.WEBVTT, StringComparison.OrdinalIgnoreCase))
             {
                 value = new VttWriter();
                 return true;

MediaBrowser.Model/MediaInfo/SubtitleFormat.cs

@@ -9,6 +9,7 @@ namespace MediaBrowser.Model.MediaInfo
         public const string SSA = "ssa";
         public const string ASS = "ass";
         public const string VTT = "vtt";
+        public const string WEBVTT = "webvtt";
         public const string TTML = "ttml";
     }
 }