Use 'media_type' instead of Rails 6.1 deprecated 'content_type'

This addresses the following Rails 6.1 deprecation notice:

DEPRECATION WARNING: Rails 6.1 will return Content-Type header without
modification. If you want just the MIME type, please use `#media_type`
instead.
Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/controllers/help_controller_spec.rb

@@ -109,7 +109,7 @@ describe HelpController do
 
         it 'renders HTML' do
           expect(response).to render_template('show.html.haml')
-          expect(response.content_type).to eq 'text/html'
+          expect(response.media_type).to eq 'text/html'
         end
       end
 
@@ -130,7 +130,7 @@ describe HelpController do
               },
               format: :png
           expect(response).to be_successful
-          expect(response.content_type).to eq 'image/png'
+          expect(response.media_type).to eq 'image/png'
           expect(response.headers['Content-Disposition']).to match(/^inline;/)
         end
       end