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'"]}
```

1 files changed, 23 insertions, 1 deletions

diff --git a/actionpack/test/assertions/response_assertions_test.rb b/actionpack/test/assertions/response_assertions_test.rb
index 17517f8965..14a04ccdb1 100644
--- a/actionpack/test/assertions/response_assertions_test.rb
+++ b/actionpack/test/assertions/response_assertions_test.rb
@@ -6,10 +6,11 @@ module ActionDispatch
     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 @@ module ActionDispatch
                    " 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