Fix back button issue

How to reproduce an error: 
* Click pjax link
* Click normal link
* Click "Back"

Expected: page should successfully load

What really happened: only pjax partial was loaded 

Reason: adding `_pjax` param to pjax request was broken. 

Why:  plugin threats `options.data` as hash `{}` but it's array: http://api.jquery.com/serializeArray/