Better way to detect CORS support

The old technique doesn't work in Firefox and
doesn't always work in Chrome depending on
when you call it.

https://hacks.mozilla.org/2009/07/cross-site-xmlhttprequest-with-cors/

Also fixes some tests that weren't overloading
the right function and were hitting the real
feature detection check

Change-Id: I0a9d6b5654efb169860ddf7e5e0551efb825920c
Merge "Better way to detect CORS support"