Only set empty handlers for IE CORS support if using XDomainRequest

[stevenp]

I ran into a mysterious issue after upgrading Axios from 0.9.1 to 0.11.0 where a Selenium test was timing out only on Firefox, and was able to bisect the issue down to this commit. I'm assuming that this change was added based on the workaround for the issue reported here.

The problem seems to be that these handlers are being set on the request, even in test mode, which seems to have an unintended effect on promise resolution in Selenium, which I assume uses these handlers, but I haven't been able to find an exact path to track down the root cause.

This PR moves the workaround into the block that sets request to an XDomainRequest, and fixes the issue. I'm assuming there isn't a reason that these should be set to empty handlers in any other case.

[coveralls]

Coverage decreased (-0.6%) to 91.254% when pulling f32b5a6 on icracked:master into ea37522 on mzabriskie:master.

[nickuraltsev]

Thank you for the PR!