Rights value must be a string in V3, but osullivan requires an array of hashes

[seanaery]

The IIIF Presentation API v3.0 spec states unambiguously that the rights property MUST be a string:
https://iiif.io/api/presentation/3.0/#rights

However, per the changes in this commit, osullivan currently raises an IIIF::V3::Presentation::IllegalValueError if you try to provide a string value. It will currently only validate if rights is an Array of hashes, each with an id key.

The presence of this bug makes it impossible to represent a Rights URI from CreativeCommons.org or RightsStatements.org in an osullivan-generated manifest and subsequently have it display in a viewer like Mirador.