Illegal Invocation fix + xhr.send() issue

Hi! I kept getting an 'Illegal invocation' error when adding headers to the xhr object so I had to fork. I also had a problem with the xhr.send() not being called if you accidentally supply a body object with a GET call.

Here are the fixes! I hope that's okay.

Abrest inspired me to write my own REST library as well. Check it out if you like!
https://github.com/leadhead9/discuss.js

Thanks,
Evan