remove needless hasOwnProperty call, allows adding options.headers

this removes `key.hasOwnProperty(options.headers)` that was always false, it should have been the other way round `options.headers.hasOwnProperty(key)`. But since this was called on a for loop over the properties of options.headers, I didn't get the reason for this. It would always be true. Or am I missing something?

I just removed this check before adding the headers to the xhr object. This allows to have headers set by passing them in `options.headers`