After saving the ORM, the resource's ETag should be updated to match the newly set value

[jcoyne]

client = Ldp::Client.new(host)
graph = RDF::Graph.new 
graph.insert([RDF::URI.new, RDF::DC.title, "foo"])
res = Ldp::Resource::RdfSource.new(client, 'res1', graph)
orm = Ldp::Orm.new(res)
orm.save
orm.graph.dump(:ttl)
=> "\n<> <http://purl.org/dc/terms/title> \"foo\" .\n"

we'd expect the dumped graph to have a real subject URI