Accept both underscore and camelcase in #update

This commit also extracts the commonality between the `update`
methods of Video and Playlist into a `Resource#update` method.
Fix tests not to pick delete videos

Tests were sometimes failing if $account.videos.first was selecting
the video that the previous test had just deleted. Ordering by
viewCount should fix the issue.
Merge pull request #46 from Fullscreen/update-accepts-both-underscore-and-camelcase

Update accepts both underscore and camelcase