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/functional/application_controller_test.rb

@@ -191,5 +191,13 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       assert_equal(1, response.parsed_body.size)
       assert_equal(tags.first.id, response.parsed_body.first.fetch("id"))
     end
+
+    should "return the correct root element name for empty xml responses" do
+      get tags_path, as: :xml
+
+      assert_response :success
+      assert_equal("tags", response.parsed_body.root.name)
+      assert_equal(0, response.parsed_body.root.children.size)
+    end
   end
 end