Fix GitHubServiceProvider.GetService<T, Ret>()

GitHubServiceProvider.GetService<T, Ret>() was previously calling
`TryGetService<T>` which had the effect of tying to cast the service got
as `T` to  an instance of `T`, which due to the joys of COM services
failed. This was breaking gists. Fixes #770.

Author: Steven Kirk

src/GitHub.VisualStudio/Services/GitHubServiceProvider.cs

@@ -224,7 +224,7 @@ public T TryGetService<T>() where T : class
         public Ret GetService<T, Ret>() where T : class
                                         where Ret : class
         {
-            return TryGetService<T>() as Ret;
+            return TryGetService(typeof(T)) as Ret;
         }
 
         public void AddService<T>(object owner, T instance) where T : class