Fix test_with_rack_env example in README

First, there was a missing quote, and second :agent is more
clear than HTTP_USER_AGENT.

README.rdoc

@@ -422,7 +422,7 @@ helper methods for testing your Sinatra app:
     end
 
     def test_with_rack_env
-      get '/', {}, 'HTTP_USER_AGENT => 'Songbird'
+      get '/', {}, :agent => 'Songbird'
       assert_equal "You're using Songbird!", @response.body
     end
   end