adding xhr header

Our error handler in express looks for `req.xhr` to be true if it's an XHR request. Currently when using FS.superagent the errors are html pages (500,503,404) since FS.superagent doesn't tell express that it's XHR.

According to http://expressjs.com/api.html#req.xhr, if we set this header, it sets the `req.xhr` to be true. I've tried setting this header in my request and it worked.