Allow status parameters of a video to be updated.

Previously Video#update could only update snippet parameters such
as title, description. After this commit, status parameters can be
updated as well.

Unfortunately, YouTube does not let users update most of the status
parameters through the API: only privacyStatus, publicStatsViewable
and publishAt can indeed be updated.

This commit also extracts the commonality between the `update`
methods of Video and Playlist into a `Resource#update` method.