Use cookies.get_cookie instead of hash_for

Since hash_for was removed in an earlier version of rack-test.
get_cookie is present in >= 2.0 and we require >=2.0.8 so we should be good

test/action_controller_test.rb

@@ -127,7 +127,7 @@ def test_getting_session_value_after_session_reset
       get '/set_session_value'
       assert_response :success
       assert cookies['_session_id']
-      session_cookie = cookies.send(:hash_for)['_session_id']
+      session_cookie = cookies.get_cookie('_session_id')
 
       get '/call_reset_session'
       assert_response :success