Make `ActionDispatch::Response#content_type` behavior configurable

I changed return value of `ActionDispatch::Response#content_type` in #36034.
But this change seems to an obstacle to upgrading. https://github.com/rails/rails/pull/36034#issuecomment-498795893

Therefore, I restored the behavior of `ActionDispatch::Response#content_type`
to 5.2 and deprecated old behavior. Also, made it possible to control the
behavior with the config.

1 files changed, 29 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/response_test.rb b/actionpack/test/dispatch/response_test.rb
index 33cf86a081..ed64d89902 100644
--- a/actionpack/test/dispatch/response_test.rb
+++ b/actionpack/test/dispatch/response_test.rb
@@ -593,4 +593,33 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_equal("text/csv", @response.media_type)
     assert_equal("utf-16", @response.charset)
   end
+
+  test "`content type` returns header that excludes `charset` when specified `return_only_media_type_on_content_type`" do
+    original = ActionDispatch::Response.return_only_media_type_on_content_type
+    ActionDispatch::Response.return_only_media_type_on_content_type = true
+
+    @app = lambda { |env|
+      if env["PATH_INFO"] == "/with_parameters"
+        [200, { "Content-Type" => "text/csv; header=present; charset=utf-16" }, [""]]
+      else
+        [200, { "Content-Type" => "text/csv; charset=utf-16" }, [""]]
+      end
+    }
+
+    get "/"
+    assert_response :success
+
+    assert_deprecated do
+      assert_equal("text/csv", @response.content_type)
+    end
+
+    get "/with_parameters"
+    assert_response :success
+
+    assert_deprecated do
+      assert_equal("text/csv; header=present", @response.content_type)
+    end
+  ensure
+    ActionDispatch::Response.return_only_media_type_on_content_type = original
+  end
 end