In xml responses, if the result is an empty array we want the response
to look like this:
<posts type="array"/>
not like this (the default):
<nil-classes type="array"/>
This refactors controllers so that this is done automatically instead of
having to manually call `@things.to_xml(root: "things")` everywhere. We
do this by overriding the behavior of `respond_with` in `ApplicationResponder`
to set the `root` option by default in xml responses.
