Defend against undefined XMLHttpRequest in _makeRequest

[jyoko]

Before submitting a pull request, please verify the following:

• If you've added code that should be tested, please add tests. (n/a)
• If you've modified the API (e.g. added a new config or public method), update the docs and TypeScript declaration file. (n/a)
• Ensure your code lints and the test suite passes (npm test).

#1023 for more info.