api: refactor default options for xml responses.

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.

app/controllers/application_controller.rb

@@ -1,6 +1,8 @@
 class ApplicationController < ActionController::Base
   class ApiLimitError < StandardError; end
 
+  self.responder = ApplicationResponder
+
   skip_forgery_protection if: -> { SessionLoader.new(request).has_api_authentication? }
   before_action :reset_current_user
   before_action :set_current_user