adding a file_set to a collection is allowed and shouldn't be

[elrayle]

To reproduce...

col = Hydra::Works::Collection.new(id: 'col-1')
fs = Hydra::Works::FileSet.new(id: 'fs-1')

col.members << fs
=> [#<Hydra::Works::FileSet id: "fs-1", head: [], tail: []>]

Should throw an error message. Instead, it incorrectly adds the file set to the collection.

[mjgiarlo]

I'm working on this one.