[fix] Fix extraHeaders option in browser (#536)

Author: lu4

lib/transports/polling-xhr.js

@@ -197,7 +197,7 @@ Request.prototype.create = function () {
     xhr.open(this.method, this.uri, this.async);
     try {
       if (this.extraHeaders) {
-        xhr.setDisableHeaderCheck(true);
+        xhr.setDisableHeaderCheck && xhr.setDisableHeaderCheck(true);
         for (var i in this.extraHeaders) {
           if (this.extraHeaders.hasOwnProperty(i)) {
             xhr.setRequestHeader(i, this.extraHeaders[i]);