Add iframe referrer headers

Fixes https://github.com/assaf/zombie/issues/739

Currently when creating an iframe, it does not send a `referrer` header when making the request to the `src`. This breaks some of google's API libraries. 

This PR sets the `referer` to the parent window's `location.href` as expected.  
It also sets the `referer` header when setting `window.location` to a different URL, which seems to be the standard behavior for all modern browsers.