fixing assignment of rdf_subjects in #attributes=

Subjects for Rdf::Resources created directly with #attributes= (i.e.
not build) were not getting set. This was causing some nodes with
assigned rdf_subjects to be treated as bnodes. Adds tests for assignment
of rdf_subjects.
Merge pull request #390 from no-reply/feature/rdf-missing-params

fixing assignment of rdf_subjects in #attributes=