Add same_site attribute and tests

[p]

The commit https://github.com/dwaite/cookiejar/commit/adb79c0a14c2b347c5289e79379a1acfe34bf388 has as its message "add support for samesite cookie". It actually only makes the validator recognize the samesite attribute, but does not expose the value via the cookie object and does not add any tests.

This PR adds the same_site attribute (note that the instance variable name was changed to @same_site from @samesite to be consistent with http_only/httponly) and adds tests.