Fix method invocations with both custom and standard param types

Prior to this fix, method calls would only work if the method's
parameters were either just standard Qt types or just custom types.
Any combination would lead to a failure invoking the method.

Change-Id: I6f140767dfec1692ba7d766284f39916057cd017
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>