Retrieving last response headers from correct place

Regardless of `_response` usage (which looks like it was meant to be private) I can tell only that Zombie itself is using it the same way to retrieve the HTTP code of last page visit in current browser window (see https://github.com/assaf/zombie/blob/6cb0b8488b2ae3e4a2a44da8ed4963687ba18a5c/src/zombie/browser.coffee#L1140).

Fixes #74