adding xhr header

Our [error handler](fs-webdev/woodruff/blob/master/lib/middleware/error-handler.js) 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) instead of returning `application/json`.

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.