Implement a 'contains?' method on event sets.

The method corresponds to the /{set}/contains/{ref}
endpoint. If the set contains the resource, it returns
the resource. Otherwise, it returns false.

@pig = Pig.create!(name: "Henwen", unique_id: "henwen")
@vision = Vision.create!(pronouncement: "In an ominous tower...")

@pig.visions.contains? @vision # => false

@pig.visions.add @vision
@pig.visions.contains? @vision # => @vision

@pig.visions.remove @vision
@pig.visions.contains? @vision # => false