use a lookup table for `assert_response`

We shouldn't depend on specific methods imlemented in the TestResponse
subclass because the response could actually be a real response object.

In the future, we should either push the aliased predicate methods in
TestResponse up to the real response object, or remove them

actionpack/lib/action_dispatch/testing/assertions/response.rb

@@ -3,6 +3,13 @@ module ActionDispatch
   module Assertions
     # A small suite of assertions that test responses from \Rails applications.
     module ResponseAssertions
+      RESPONSE_PREDICATES = { # :nodoc:
+        success:  :successful?,
+        missing:  :not_found?,
+        redirect: :redirection?,
+        error:    :server_error?,
+      }
+
       # Asserts that the response is one of the following types:
       #
       # * <tt>:success</tt>   - Status code was in the 200-299 range
@@ -20,11 +27,9 @@ module ResponseAssertions
       #   # assert that the response code was status code 401 (unauthorized)
       #   assert_response 401
       def assert_response(type, message = nil)
-        message ||= "Expected response to be a <#{type}>, but was <#{@response.response_code}>"
-
         if Symbol === type
           if [:success, :missing, :redirect, :error].include?(type)
-            assert @response.send("#{type}?"), message
+            assert_predicate @response, RESPONSE_PREDICATES[type], message
           else
             code = Rack::Utils::SYMBOL_TO_STATUS_CODE[type]
             if code.nil?

actionpack/test/assertions/response_assertions_test.rb

@@ -7,7 +7,7 @@ class ResponseAssertionsTest < ActiveSupport::TestCase
       include ResponseAssertions
 
       FakeResponse = Struct.new(:response_code) do
-        [:success, :missing, :redirect, :error].each do |sym|
+        [:successful, :not_found, :redirection, :server_error].each do |sym|
           define_method("#{sym}?") do
             sym == response_code
           end
@@ -16,7 +16,7 @@ class ResponseAssertionsTest < ActiveSupport::TestCase
 
       def test_assert_response_predicate_methods
         [:success, :missing, :redirect, :error].each do |sym|
-          @response = FakeResponse.new sym
+          @response = FakeResponse.new RESPONSE_PREDICATES[sym].to_s.sub(/\?/, '').to_sym
           assert_response sym
 
           assert_raises(Minitest::Assertion) {