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.

test/test_helper.rb

@@ -113,6 +113,8 @@ class ActionDispatch::IntegrationTest
     super
     Socket.stubs(:gethostname).returns("www.example.com")
     Danbooru.config.stubs(:enable_sock_puppet_validation?).returns(false)
+
+    ActionDispatch::IntegrationTest.register_encoder :xml, response_parser: ->(body) { Nokogiri.XML(body) }
   end
 
   def teardown