fix(TestAgent): attach cookies to agent after plugin is used

following superagent:
    PR: ladjs/superagent#1556
commit: ladjs/superagent@4babc5d

Author: lamweili

Diff for: lib/agent.js

@@ -55,8 +55,8 @@ methods.forEach(function(method) {
     req.on('response', this._saveCookies.bind(this));
     req.on('redirect', this._saveCookies.bind(this));
     req.on('redirect', this._attachCookies.bind(this, req));
-    this._attachCookies(req);
     this._setDefaults(req);
+    this._attachCookies(req);
 
     return req;
   };