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
return toString.call(val) === '[object Blob]';
}
/**
 * Determine if a value is a Function
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is a Function, otherwise false
 */
function isFunction(val) {
  return toString.call(val) === '[object Function]';
}
/**
 * Determine if a value is a Stream
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is a Stream, otherwise false
 */
function isStream(val) {
  return isObject(val) && isFunction(val.pipe);
}
/**
 * Determine if a value is a URLSearchParams object
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is a URLSearchParams object, otherwise false
 */
function isURLSearchParams(val) {