Implementation for assertSubset #1371

I probably will not have time to open this PR later this weekend so I decided to do it now, just in case the feature discussed at #1371 get's approved :D

In case it gets approved but with a different assertion name I'll be happy to update the PR, just ping me.

Cheers.
@marcioAlmada Alright, I should have time to review/merge this later today. Thanks :)