Improve error messages in cookies_test

Using `assert_predicate` and `assert_match` instead of just `assert` is
preferrable because better error messages are output.

In the case of `assert response.cookies.empty?` the error message was
`Failed assertion, no message given.` but now with `assert_predicate` it
will be `Expected {"user_name"=>"david"} to be empty?.`

For `assert_match(/user_name=david/,
response.headers["Set-Cookie"])` as well, the message returned was
unhelpful - `Failed assertion, no message given.` but now will tell what
was expected and what was returned with `Expected /user_name=david/ to
match "user_name=nope; path=/".`

actionpack/test/dispatch/cookies_test.rb

@@ -280,7 +280,7 @@ def test_setting_cookie
   def test_setting_the_same_value_to_cookie
     request.cookies[:user_name] = 'david'
     get :authenticate
-    assert response.cookies.empty?
+    assert_predicate response.cookies, :empty?
   end
 
   def test_setting_the_same_value_to_permanent_cookie
@@ -360,7 +360,7 @@ def test_delete_cookie_with_path
   def test_delete_unexisting_cookie
     request.cookies.clear
     get :delete_cookie
-    assert @response.cookies.empty?
+    assert_predicate @response.cookies, :empty?
   end
 
   def test_deleted_cookie_predicate
@@ -378,7 +378,7 @@ def test_deleted_cookie_predicate_with_mismatching_options
 
   def test_cookies_persist_throughout_request
     response = get :authenticate
-    assert response.headers["Set-Cookie"] =~ /user_name=david/
+    assert_match(/user_name=david/, response.headers["Set-Cookie"])
   end
 
   def test_set_permanent_cookie