Even more reuse of @_
Make sure is_plain_value returns the actual object pre-stringify

This is important since DBIC is starting to heavily rely on the
contents of the returned []
Extra tests that I can *not* make fail, oh well...

This is an attempt to simulate the behavior of JSON::PP::Boolean, and to
replicate the error autarch was seeing - clearly there is more to it than
just simple value juggling. Oh well, commit already written tests anyway.

See further commits for the actual code changes