webdriver: response: add single cookie response type (#102)

* response: add single cookie response type

The WebDriver specification mandates that the GetNamedCookie command
should return a cookie serialisation object, and not an array of
cookies.

This patch introduces WebDriverResponse::Cookie and CookieResponse types
to allow WebDriver implementations to return single cookie objects.

Fixes: mozilla/geckodriver#463
Source-Repo: https://github.com/mozilla/webdriver-rust
Source-Revision: 9b49e5a6ec370e0da09521bfedcc596427c93cd5

committer: jgraham <jameshoppipolla.co.uk>

UltraBlame original commit: ac919ddfbcc3db7ca10c0d5219caea795527e79e