Header browserkit driver

With `"behat/mink-browserkit-driver": "~1.2@dev"` and this change https://github.com/Behat/MinkBrowserKitDriver/pull/53.

Headers setted [here](https://github.com/stephpy/behatch-contexts/blob/header_browserkit_driver/src/Context/RestContext.php#L190) are stored on `Behat\Mink\Driver\BrowserKitDriver` and no more on client. 

BTW, when we try to make a request from `behatch/rest-context`, there is no headers given to the request (since it not uses `visit` method of `Behat\Mink\Driver\BrowserKitDriver`).

I add a stack of headers on this context but a getter `getServerParameters`on `Behat\Mink\Driver\BrowserKitDriver` would be better imho. Any idea about this ? ping @stof @sanpii


ps: This PR is only for users which uses `behat/mink-browserkit-driver: ~1.2@dev` so we should not merge it NOW (before mink-browserkit-driver is released)