Change `ActionDispatch::Response#content_type` returning Content-Type header as it is

Since #35709, `Response#conten_type` returns only MIME type correctly.
It is a documented behavior that this method only returns MIME type, so
this change seems appropriate.
https://github.com/rails/rails/blob/39de7fac0507070e3c5f8b33fbad6fced84d97ed/actionpack/lib/action_dispatch/http/response.rb#L245-L249

But unfortunately, some users expect this method to return all
Content-Type that does not contain charset. This seems to be breaking
changes.

We can change this behavior with the deprecate cycle.
But, in that case, a method needs that include Content-Type with
additional parameters. And that method name is probably the
`content_type` seems to properly.

So I changed the new behavior to more appropriate `media_type` method.
And `Response#content_type` changed (as the method name) to return Content-Type
header as it is.

Fixes #35709.

[Rafael Mendonça França & Yuuji Yaginuma ]

3 files changed, 27 insertions, 1 deletions

diff --git a/guides/source/6_0_release_notes.md b/guides/source/6_0_release_notes.md
index fa45e7240d..e421ae1ac7 100644
--- a/guides/source/6_0_release_notes.md
+++ b/guides/source/6_0_release_notes.md
@@ -215,6 +215,10 @@ Please refer to the [Changelog][action-pack] for detailed changes.
 
 ### Notable changes
 
+*   Change `ActionDispatch::Response#content_type` returning Content-Type
+    header as it is.
+    ([Pull Request](https://github.com/rails/rails/pull/36034))
+
 *   Raise an `ArgumentError` if a resource param contains a colon.
     ([Pull Request](https://github.com/rails/rails/pull/35236))
 
diff --git a/guides/source/testing.md b/guides/source/testing.md
index 9540bb2af5..41bc54b924 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -1144,7 +1144,7 @@ test "ajax request" do
   get article_url(article), xhr: true
 
   assert_equal 'hello world', @response.body
-  assert_equal "text/javascript", @response.content_type
+  assert_equal "text/javascript", @response.media_type
 end
 ```
 
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index b8a5c39f39..1110592d5e 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -134,6 +134,28 @@ Action Cable JavaScript API:
   +    ActionCable.logger.enabled = false
   ```
 
+### `ActionDispatch::Response#content_type` now returned Content-Type header as it is.
+
+Previously, `ActionDispatch::Response#content_type` returned value does NOT contain charset part.
+This behavior changed to returned Content-Type header containing charset part as it is.
+
+If you want just MIME type, please use `ActionDispatch::Response#media_type` instead.
+
+Before:
+
+```ruby
+resp = ActionDispatch::Response.new(200, "Content-Type" => "text/csv; header=present; charset=utf-16")
+resp.content_type #=> "text/csv; header=present"
+```
+
+After:
+
+```ruby
+resp = ActionDispatch::Response.new(200, "Content-Type" => "text/csv; header=present; charset=utf-16")
+resp.content_type #=> "text/csv; header=present; charset=utf-16"
+resp.media_type   #=> "text/csv"
+```
+
 ### Autoloading
 
 The default configuration for Rails 6