fix: correct OPTS error code (#190)

If an unknown option is given, the response should be 501

Author: trs

src/commands/registration/opts.js

@@ -13,7 +13,7 @@ module.exports = {
     const [_option, ...args] = command.arg.split(' ');
     const option = _.toUpper(_option);
 
-    if (!OPTIONS.hasOwnProperty(option)) return this.reply(500);
+    if (!OPTIONS.hasOwnProperty(option)) return this.reply(501, 'Unknown option command');
     return OPTIONS[option].call(this, args);
   },
   syntax: '{{cmd}}',