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/logical/application_responder.rb

@@ -0,0 +1,12 @@
+# https://github.com/plataformatec/responders
+# https://github.com/plataformatec/responders/blob/master/lib/action_controller/responder.rb
+class ApplicationResponder < ActionController::Responder
+  # this is called by respond_with for non-html, non-js responses.
+  def to_format
+    if format == :xml
+      options[:root] ||= resource.table_name.dasherize if resource.respond_to?(:table_name)
+    end
+
+    super
+  end
+end