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.

As a side-note, it might make sense to directly call [Frame.create](https://github.com/assaf/zombie/blob/763845908e60bf8077d3edb0521b10bf6792fa54/src/zombie/dom_iframe.coffee#L26) with the `url` when creating an iframe using a `src` attribute. Currently one is created pointing to `about:blank`, then it's `window.location` is set to the `src` later, which complicates things a bit.