Regression on 2.3 branch: Can't delete cookies from inside a test

This commit introduced a regression: http://github.com/brynary/rails/commit/143c55d325388aa2384de8f137cf...

This test (included in my branch) demonstrates the behavior change:

def test_cookies_are_clearable_from_test
  get :authenticate
  cookies.clear
  response = get :count
  assert_equal "0", response.body
end

With 143c55d3 applied, the "count" action sees 1 cookie instead of 0. The following branch reverts the changeset and adds a test for the behavior that broke: http://github.com/brynary/rails/commits/cookie-regression