Test for mobile UI single post view

Verify that the correct layout is rendered (rather than no layout
at all) in the mobile UI single post view.

This is a test I forgot to write at 5am when I was so excited about
having the mobile UI working on my Blackberry again (bad me!), so
here it is before it slips my mind. 😁

This tests the change made to posts_controller in 6abe718227

spec/controllers/posts_controller_spec.rb

@@ -30,6 +30,11 @@ describe PostsController do
         response.should be_success
       end
 
+      it 'renders the application layout on mobile' do
+        get :show, :id => @message.id, :format => :mobile
+        response.should render_template('layouts/application')
+      end
+
       it 'succeeds on mobile with a reshare' do
         get :show, "id" => Factory(:reshare, :author => alice.person).id, :format => :mobile
         response.should be_success