[fix] Use xhr.responseText if xhr.response is not provided (#483)

Author: miguelgrinberg

lib/transports/polling-xhr.js

@@ -341,7 +341,7 @@ Request.prototype.onLoad = function () {
       contentType = this.xhr.getResponseHeader('Content-Type').split(';')[0];
     } catch (e) {}
     if (contentType === 'application/octet-stream') {
-      data = this.xhr.response;
+      data = this.xhr.response || this.xhr.responseText;
     } else {
       if (!this.supportsBinary) {
         data = this.xhr.responseText;