Supertest Syntax for Not-Set HTTP Header

How do I not set an HTTP header with Superagent, and how do I then test that the header is not set with Supertest? Do I set it to an empty string, or undefined, or null, or what? For example, in this jQuery function, how would I not set the header in the else block:

if(this.true == true) {
    request
        .post('/page')
        .set('X-XSS-Protection', '1; mode=block');
else {
    request
        .post('/page')
        .set('X-XSS-Protection', /*what do I put here?*/);

And, with Buster.js in this case, how do I test it?

request = require('supertest');
express = require('express');
server = express();

request(server)
    .get('/page')
    .expect(/*what do I put here?*/);