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/comments_controller.rb

@@ -78,11 +78,7 @@ private
     @posts = @posts.includes(comments: [:creator])
     @posts = @posts.includes(comments: [:votes]) if CurrentUser.is_member?
 
-    respond_with(@posts) do |format|
-      format.xml do
-        render :xml => @posts.to_xml(:root => "posts")
-      end
-    end
+    respond_with(@posts)
   end
 
   def index_by_comment
@@ -91,9 +87,6 @@ private
       format.atom do
         @comments = @comments.includes(:post, :creator).load
       end
-      format.xml do
-        render :xml => @comments.to_xml(:root => "comments")
-      end
     end
   end