Building a REST API using node & Express.js, my PUT method doesn't work for some reason. Can anyone help me please?

My data model :

var UserDetail = new Schema({
      username: String,
      password: String
    }, {
      collection: 'userInfo'
    });
var UserDetails = mongoose.model('userInfo', UserDetail);

This is my post method in the routes file

//POST
router.post( '/api/users', function( req, res ) {
    var user = new UserDetails({
        username: req.body.username,
        password: req.body.password
    });
    console.log(req.body.username);
    user.save( function( err ) {
        if( !err ) {
            console.log( 'created' );
            return res.send( user );
        } else {
            console.log( err );
            return res.send('ERROR');
        }
    });
});

My get api works perfect. The josn looks like this :

{"username":"exmpleUsername","password":"examplePassword","_id":"54091d9df8f00fb42055b6f8","__v":0}

But when I try to POST using POSTMAN, it doesn't add the username and password. It just adds new _id like this:

{"_id":"540e394f37706f701020cf19","__v":0}

Thanks in advance!