Difference in persister behavior when handling array defaults

[dlpierce]

While working on valkyrie/ferdora support in hyrax, I've discovered a difference in the way the various persisters handle the default value for an attribute of type array (a frozen empty array).

After persisting a new resource, or when reloading it, the postgres and memory persisters produce a resource with an unfrozen array, while the fedora and solr persisters the array is frozen. Based on past work in valkyrie, I believe the intention is for the default value for arrays to be frozen for all of the persistence adapters. See this branch which has a spec added for this case, but no failures for fedora and solr.