Avoid use of Hash#dig to keep compatibility with Ruby 2.1

app/controllers/concerns/oauth_applications.rb

@@ -6,7 +6,7 @@ module OauthApplications
   end
 
   def prepare_scopes
-    scopes = params.dig(:doorkeeper_application, :scopes)
+    scopes = params.fetch(:doorkeeper_application, {}).fetch(:scopes, nil)
 
     if scopes
       params[:doorkeeper_application][:scopes] = scopes.join(' ')