make Upload a generic POST

The Releases API-specific part of uploading (the accept header) is now
in the Releases client and the API connection just has a generic POST
method. /cc @spraints

Octokit/Http/IApiConnection.cs

@@ -22,6 +22,6 @@ namespace Octokit
         Task<T> Update<T>(Uri endpoint, object data);
         [SuppressMessage("Microsoft.Design", "CA1004:GenericMethodsShouldProvideTypeParameter", Justification="Legitimate, but I'm not fixing it just yet.")]
         Task Delete<T>(Uri endpoint);
-        Task<T> Upload<T>(Uri uri, Stream rawData, string contentType);
+        Task<T> Post<T>(Uri uri, Stream rawData, string contentType, string accepts);
     }
 }