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_flags_controller.rb

@@ -10,11 +10,7 @@ class PostFlagsController < ApplicationController
   def index
     @post_flags = PostFlag.search(search_params).includes(:creator, post: [:flags, :uploader, :approver])
     @post_flags = @post_flags.paginate(params[:page], limit: params[:limit])
-    respond_with(@post_flags) do |format|
-      format.xml do
-        render :xml => @post_flags.to_xml(:root => "post-flags")
-      end
-    end
+    respond_with(@post_flags)
   end
 
   def create