Objects loaded from Solr should be read-only. Fixes #555
Merge pull request #603 from projecthydra/read_only

Objects loaded from Solr should be read-only. Fixes #555
Consistent use of RDF constant

Renames instances of "Rdf" to "RDF" to keep it consistent with other
classes.
Merge pull request #604 from projecthydra/rdf

Consistent use of RDF constant
Translate property to predicate in Builder::Association
Fixes #597