Improve assert_response helper

When the check is failed, print the actual response body if it's not too large.
This could improve productivity when writing new tests.

Before:

```
ThemeEditorIntegrationTest#test_whatever
    Expected response to be a <200: ok>, but was a <422: Unprocessable Entity>.
Expected: 200
  Actual: 422
```

After:

```
ThemeEditorIntegrationTest#test_whatever
    Expected response to be a <200: ok>, but was a <422: Unprocessable Entity>.
Expected: 200
  Actual: 422
Response body: {"errors":["Invalid settings object for section '1'"]}
```

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

@@ -79,7 +79,12 @@ def normalize_argument_to_redirection(fragment)
         def generate_response_message(expected, actual = @response.response_code)
           "Expected response to be a <#{code_with_name(expected)}>,"\
           " but was a <#{code_with_name(actual)}>"
-          .concat location_if_redirected
+            .concat(location_if_redirected).concat(response_body_if_short)
+        end
+
+        def response_body_if_short
+          return "" if @response.body.size > 500
+          "\nResponse body: #{@response.body}"
         end
 
         def location_if_redirected

actionpack/test/assertions/response_assertions_test.rb

@@ -6,10 +6,11 @@ module Assertions
     class ResponseAssertionsTest < ActiveSupport::TestCase
       include ResponseAssertions
 
-      FakeResponse = Struct.new(:response_code, :location) do
+      FakeResponse = Struct.new(:response_code, :location, :body) do
         def initialize(*)
           super
           self.location ||= "http://test.example.com/posts"
+          self.body ||= ""
         end
 
         [:successful, :not_found, :redirection, :server_error].each do |sym|
@@ -112,6 +113,27 @@ def test_error_message_shows_302_redirect_when_302_asserted_for_301
                    " redirect to <http://test.host/posts/redirect/2>"
         assert_match expected, error.message
       end
+
+      def test_error_message_shows_short_response_body
+        @response = ActionDispatch::Response.new
+        @response.status = 400
+        @response.body = "not too long"
+        error = assert_raises(Minitest::Assertion) { assert_response 200 }
+        expected = "Expected response to be a <200: OK>,"\
+                   " but was a <400: Bad Request>" \
+                   "\nResponse body: not too long"
+        assert_match expected, error.message
+      end
+
+      def test_error_message_does_not_show_long_response_body
+        @response = ActionDispatch::Response.new
+        @response.status = 400
+        @response.body = "not too long" * 50
+        error = assert_raises(Minitest::Assertion) { assert_response 200 }
+        expected = "Expected response to be a <200: OK>,"\
+                   " but was a <400: Bad Request>"
+        assert_match expected, error.message
+      end
     end
   end
 end