Pass statistics and content details to Video#new

If a user specifies .where(part: 'statistics') when listing videos,
then YouTube will return the "statistics" part and the Video object
will be initialized with that data. Same for content details.

This **overrides the default behavior** of including status and snippet,
so if those parts are still required, the request should be:

    .where(part: 'statistics,snippet,status,contentDetails')

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This partially solves the issue tackled by #84 adding missing specs and moving code from "Resources" to "Videos", since not all resources have statistics and content details (e.g., playlists and playlist items don't).

I think the general issue raised by #84 is still relevant (to add an `.include` method, rather than passing the parts to `.where`), but for the sake of unblocking that issue, I suggest for this PR to be merged.

Note that extending the behavior of this PR from Videos to Channels can be done by… duplicating the code :scream: – not a big deal because these methods are being rewritten in a different branch :blush: