fix adding error on ruby 3.0

[mkrajewski90]

After updating to ruby 3.0, when adding errors there are an error. It's easy to fix - it requires to set explicity key on options.

[Annedj]

Could instead do record.errors.add(attribute, :undisposable, **options) as the documentation shows

[mkrajewski90]

changed

[jarthod]

Thanks @mkrajewski90, for the record I merged this fix in my improved branch (#7)