Clear metadata provider cache when provider parts are registered

AddParts replaces _metadataProviders but left _metadataProviderCache
holding provider arrays from the previous registration. Invalidate the
cache so repeated AddParts calls cannot serve stale providers, and give
the previously uncalled ClearMetadataProviderCache its intended caller.

MediaBrowser.Providers/Manager/ProviderManager.cs

@@ -163,6 +163,8 @@ namespace MediaBrowser.Providers.Manager
             _externalUrlProviders = externalUrlProviders.OrderBy(i => i.Name).ToArray();
 
             _savers = metadataSavers.ToArray();
+
+            ClearMetadataProviderCache();
         }
 
         /// <inheritdoc/>