Add User-Agent header to CORS preflight requests
Add User-Agent header to CORS preflight requests

Fixes #1659.

This mirrors the behavior of other browsers, and fixes broken XHR
requests to APIs that require a User-Agent header for all requests,
such as api.github.com.

mzgol/jsdom,tmpvar/jsdom,mzgol/jsdom,tmpvar/jsdom,mzgol/jsdom