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/post_versions_controller.rb

@@ -5,11 +5,7 @@ class PostVersionsController < ApplicationController
 
   def index
     @post_versions = PostArchive.includes(:updater, post: [:versions]).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
-    respond_with(@post_versions) do |format|
-      format.xml do
-        render xml: @post_versions.to_xml(root: "post-versions")
-      end
-    end
+    respond_with(@post_versions)
   end
 
   def search