Don't wipe out gon data upon AJAX request

When I render a page with some gon data
And I send an AJAX request to another controller action (name it update_gon) for additional gon data
And I have `= include_gon` in the update_gon.html.haml, which is rendered without layout
The previous javascript gon variables should not be wiped out

Clearing of `window.gon` should occur only with first inclusion, or, as I propose, make it optional.