Remove headers entirely to avoid XDR error

Even "no headers" is seen as setting headers to IE.

src/mixins/api.jsx

@@ -30,7 +30,7 @@ var Api = {
                 // For IE < 10, we must use XDR for cross-domain requests. XDR does not support
                 // custom headers.
                 defaults(opts, {useXDR: true});
-                opts.headers = {};
+                delete opts.headers;
             }
             xhr(opts, function (err, res, body) {
                 if (err) log.error(err);