Triggering CORS pre-flighting on firefox

[DanielHeath]

The x-requested-with header set in https://github.com/jedmao/iso-http/blob/master/lib/browser/Http.ts#L18 causes firefox to perform a CORS request where it otherwise wouldn't need to.

This adds ~100ms to XHR requests for mobile users. Does it make sense for clients that want this behavior to specify it themselves rather than baking it into the library?

[DanielHeath]

http://stackoverflow.com/questions/8163703/cross-domain-ajax-doesnt-send-x-requested-with-header explains what jQuery does in the same situation and why.