play nicer with bundles that use mvp args

The documentation says:

    NOTE: If you overwrite mvp_multivalue_args you'll need to include the
    value of plugin_remover_attribute (-remove by default) if you want to
    retain this functionality. As always, patches and suggestions are welcome.

We can avoid having to do this in the composing class by using a method
modifier, with a fallback method (to wrap in case the class doesn't define
one) that returns nothing.