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 typeof URLSearchParams !== 'undefined' && val instanceof URLSearchParams;
}
/**
 * Trim excess whitespace off the beginning and end of a string
 *
 * @param {String} str The String to trim
 * @returns {String} The String freed of excess whitespace
 */
function trim(str) {
  return str.replace(/^\s*/, '').replace(/\s*$/, '');
}
/**
 * Determine if we're running in a standard browser environment
 *
 * This allows axios to run in a web worker, and react-native.
 * Both environments support XMLHttpRequest, but not fully standard globals.
 *
 * web workers:
 *  typeof window -> undefined
 *  typeof document -> undefined
 *
 * react-native:
 *  navigator.product -> 'ReactNative'
 * nativescript
 *  navigator.product -> 'NativeScript' or 'NS'
 */
function isStandardBrowserEnv() {