added additional in-line unit tests and fixes for Join(..) operation.
added additional in-line unit tests and fixes for Join(..) operation.

git-svn-id: 60eb48102d1ca0c284ab3654bef916b01e1b221b@1137 4f84a9ca-5c3e-0410-8a68-0bdf370325d5

nwebs/rdflib