don't mutate a response object after to_a

When the response object is `to_a`'d, that means it's been written to
the socket.  It doesn't make sense to mutate the response object after
it's been written (and this may raise an exception in the future).

actionpack/test/dispatch/response_test.rb

@@ -150,6 +150,8 @@ def test_only_set_charset_still_defaults_to_text_html
     assert_equal "user_name=david; path=/", headers["Set-Cookie"]
     assert_equal({"user_name" => "david"}, @response.cookies)
 
+    @response = ActionDispatch::TestResponse.new
+    @response.set_cookie("user_name", :value => "david", :path => "/")
     @response.set_cookie("login", :value => "foo&bar", :path => "/", :expires => Time.utc(2005, 10, 10,5))
     status, headers, body = @response.to_a
     assert_equal "user_name=david; path=/\nlogin=foo%26bar; path=/; expires=Mon, 10 Oct 2005 05:00:00 -0000", headers["Set-Cookie"]