A new pattern for adding multi-registration services

This is some cleanup of how we add multi-registration services to avoid
duplication when calling AddMvcServices multiple times. Also improved
tests to be more clear, and to verify all of our special cases
explicitly.

src/Microsoft.AspNet.Mvc/MvcServiceCollectionExtensions.cs

@@ -417,4 +417,4 @@ namespace Microsoft.Framework.DependencyInjection
                 routeOptions => routeOptions.ConstraintMap.Add("exists", typeof(KnownRouteValueConstraint)));
         }
     }
-}
+}