Adding error for bad nested attribute arguments

Sending a bad argument to Rdf::Resource#attributes= previously failed
silently. This is a problem, since it makes modeling errors invisible to
nested attributes setters, leading to missing data. Other Rails nested
attributes seem to throw an ArgumentError. This PR mimics that behavior
and message.

Closes #393.
Merge pull request #395 from no-reply/feature/nested-attribute-argument-errors

Adding error for bad nested attribute arguments