Resolver reports the wrong type when a dynamic call has only one applicable method.

When invoking a method with dynamic arguments (and there is only one applicable method), convert the result of the call to 'dynamic' (http://blogs.msdn.com/b/ericlippert/archive/2012/10/22/a-method-group-of-one.aspx).

This fix might appear to decrease the IDE experience since it will cause code that has no chance of working to be reported witout errors, but it is in accordance with the C# spec.