Revert "Rdf_Datastream#content= probably shouldn't set #new? to false, if the resource is unpersisted. It probably also shouldn't persist. (is this test really necessary?)"

Looks like this test is correct, if confusing; better patch needed.

This reverts commit 1a1f1e80188d2f8c4b21ffb67fc65a94c9c5ea9a.
some tidying up