More robust handling contexts in SPARQLStore

Previously, queries via contexts of a Conjunctive graph have been handled by injecting a `GRAPH` clause into the query. This does not look very robust. The right way is to use the URL parameter `default-graph-uri`. For updates, `using-graph-uri` can be used. (Up to now, update was not able to handle contexts.)

I tried to implement both, but for reasons unknown to me, some tests in test_sparqlupdatestore still fail. (Maybe my Sesame endpoint is buggy?) I will try again a later day.

Does anyone know of any SPARQL endpoints out there which do not yet support `default-graph-uri` (is already part of SPARQL 1) and `using-graph-uri`?

By the way, I also fixed a bug in Graph.update.