Response Headers don't overwrite cookies

- set initial cookie before request
  page.driver.set_cookie("user_id", 1)
- controller sets cookies[:user_id] = 2
  visit "/change_user?user_id=2" 
- this passes - the right headers were sent back by rails app
  page.response_headers["Set-Cookie"].match("user_id=2").should_not be_nil
- this fails, user_id cookie is still 1
  page.driver.cookies["user_id"].value.should == 2
