Option preflightContinue not working with origin function

[RiccardoRomagnoli]

When origin has a function logic it prevents to be overwritten on a next() by another cors policy when preflightContinue is set true.

Example:

const allowedOrigins = [`${config.frontendUrl}`];
// enable cors
app.use(cors({
  origin: function(origin, callback){
    if(!origin) return callback(null, true);
    if(allowedOrigins.indexOf(origin) === -1){
      const msg = 'The CORS policy for this site does not ' +
                'allow access from the specified Origin.';
      return callback(new Error(msg), false);
    }
    return callback(null, true);
  },
  credentials: true,
  // Allow follow-up middleware to override this CORS for options
  preflightContinue: true,
}));

This should be applied everywhere exept if redefined like

const corsOptions = { 
  origin: true,
  credentials: true
}
router.post('/test', cors(corsOptions), (req, res) => {});

In this example /test will always be blocked by cors policy, if instead I use origin: allowedOrigins (Array value) it works. Any ideas?

#40