**Describe the bug**
Running Jest unit tests with xhr-mock https://github.com/jameslnewell/xhr-mock fail. Reverting to 0.18.0 fixed it.
lib/defaults.js
'use strict';
var utils = require('./utils');
var normalizeHeaderName = require('./helpers/normalizeHeaderName');
var DEFAULT_CONTENT_TYPE = {
  'Content-Type': 'application/x-www-form-urlencoded'
};
function setContentTypeIfUnset(headers, value) {
  if (!utils.isUndefined(headers) && utils.isUndefined(headers['Content-Type'])) {
    headers['Content-Type'] = value;
  }
}
function getDefaultAdapter() {
  var adapter;
  
  if (typeof process !== 'undefined' && Object.prototype.toString.call(process) === '[object process]') {
    
    adapter = require('./adapters/http');
  } else if (typeof XMLHttpRequest !== 'undefined') {
    
    adapter = require('./adapters/xhr');
  }
  return adapter;
}
var defaults = {