Don't fail on add_video if the video is forbidden

An example is https://www.youtube.com/watch?v=kDCpdKeTe5g

"This video is no longer available because the YouTube account associated with this video has been terminated."
Merge branch 'don-t-fail-on-adding-unavailable-videos'
Fix failing test on environment variable

Once an ENV is set in a previous test, it must be explicitly set
to nil, otherwise it keeps its value.
Merge branch 'fix-failing-test'
No need to have collections inherit from Resources

One superclass for all collections is enough, and it's called Base.
Merge branch 'remove-collection-resources'
Playlist#update ONLY changes the specified attrs

By default, the YouTube API method to update a playlist would also
change to "nil" all the attributes that are not explicitly given.

This behavior is counter-intuitive and dangerous
Merge branch 'only-update-passed-parameters'
[skip ci] @wip Need custom RequestError before proceeding

I am separating Account's credential into CredentialsSet.

After that, I'll try to use the same has_one abstraction