config: flip action_dispatch_return_only_media_type_on_content_type

This makes `ActionDispatch::Controller#content_type` return not only
the MIME type, but also in some circumstances the charset.

Example:

```ruby
reponse.content_type == 'text/html; charset=utf-8'
```

The MIME type-only fragment can now be accessed using `#media_type`.

Changes to the tests are not stricly necessary (because no charset is
present in the actual value), but represent the intent better.

spec/controllers/instructeurs/procedures_controller_spec.rb

@@ -540,8 +540,8 @@ describe Instructeurs::ProceduresController, type: :controller do
           format: :js
       end
 
-      it "responses in the correct format" do
-        expect(response.content_type).to eq "text/javascript"
+      it 'responds in the correct format' do
+        expect(response.media_type).to eq('text/javascript')
         expect(response).to have_http_status(:ok)
       end
     end

spec/controllers/new_administrateur/attestation_templates_controller_spec.rb

@@ -23,7 +23,7 @@ describe NewAdministrateur::AttestationTemplatesController, type: :controller do
     render_views
     it 'renders a PDF' do
       expect(subject.status).to eq(200)
-      expect(subject.content_type).to eq('application/pdf')
+      expect(subject.media_type).to eq('application/pdf')
     end
   end