URL preserving option

Allows to preserve URL until response is received.
For example, on some `http://site.com/blog` is a link `http://site.com/refresh` which jush redirects back to `http://site.com/blog`.
If user clicks on `refresh` with pjax URL will change to `http://site.com/refresh` for a bit and then (on response) back to `http://site.com/blog`.
With this option URL will remain same during all refreshing.