Ensure errors are always returned in JSON in API

app/controllers/api/application_controller.rb

@@ -37,7 +37,9 @@ module API
     end
 
     def json_filter!
-      head :not_acceptable if request.format != :json
+      if request.format != :json
+        head :not_acceptable, content_type: 'application/json'
+      end
     end
   end
 end

spec/integration/api/application_spec.rb

@@ -15,6 +15,7 @@ describe 'API application' do
     it 'responds with a 406 when request format is not JSON' do
       get api_ping_path, format: :xml
       expect(response.status).to be 406
+      expect(response.content_type).to eq :json
       expect(response.body).to be_empty
     end
   end