Missing default_graph in sparql_protocol writer

[jurrian]

When writing to Openlink Virtuoso (07.20.3217) with the current sparql_protocol writer, a 400 bad request is returned with the following error:

Virtuoso 37000 Error SP031: SPARQL compiler: No plain default graph specified in the preamble, but it is needed for triple constructor in DELETE {...} without GRAPH {...}

Examples of the sparql_protocol feature a default_graph attribute:

store = surf.Store(reader = 'sparql_protocol',
                   endpoint = 'http://dbpedia.org/sparql',
                   default_graph = 'http://dbpedia.org')

However, in the code there seems no sign of usage of this attribute, which is needed for triple stores like Virtuoso.

I will create a pull request for the fix.

[jurrian]

Pull request merged, closing this issue