If a header has been set as a default, there does not appear to be any way to skip it on an individual request. Setting `null` or `undefined` doesn't do anything.

lib/utils.js
'use strict';
var bind = require('./helpers/bind');
var toString = Object.prototype.toString;
/**
 * Determine if a value is an Array
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is an Array, otherwise false
 */
function isArray(val) {
  return toString.call(val) === '[object Array]';
}
/**
 * Determine if a value is undefined
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if the value is undefined, otherwise false
 */
function isUndefined(val) {