fix close function to abort XHR

[brian-medendorp]

I noticed while using the polyfill, calling the close() function was not actually closing the http connection, but making this small change to call the abort() function of the actual xhr reference fixed the problem.

[brian-medendorp]

I did attempt to write tests for this, but I couldn't find any existing tests of the close function, except for an assertion of the readyState after close() is called. There's no external reference to "req" or "xhr", so writing a test would likely involve mocking the entire http/https dependency, which seems a bit excessive for one line of code.

[aslakhellesoy]

Thanks!

Yeah, it's harder to write tests for the polyfill - would be nice to have some automated tests running in an actual browser.