Add tests for issue #1299

[aucampia]

These tests confirm that bound namespaces occur
exactly once in SPARQL queries when using SPARQLStore.

Closes #1299 (Issue does not exist, this just adds tests to confirm it does not exists)

Proposed Changes

• Check that query argument to rdflib.plugins.stores.sparqlstore.SPARQLConnector.query contains bound namespaces exactly once.
• Add Mock HTTP server based tests for queries to SPARQLStore that confirms the query send to the server contains bound namespaces exactly once.

I think the mock server based test is better, and ideally this should replace all the test that queries http://dbpedia.org/sparql, but it adds about 0.5 seconds to tests as it stands, so maybe it is not ideal. If you want only one of the two approaches let me know and I will remove the other.

[coveralls]

Coverage decreased (-0.08%) to 75.627% when pulling 9659a2e on iafork:issue1299_tests into 06df8b6 on RDFLib:master.

[nicholascar]

This looks really great: we have to do something about our testing dependency on DBPedia and others, so having a mock server set up here could likely be the pattern we want to use for all SPARQL testing.

I'll review this properly over the weekend (looks good anyway) and see if I can reuse the pattern for a couple of other tests. If so, I'll try and assess how widely this pattern can apply and what utility functions we might make for multiple test to use.

Again @aucampia, thanks for the great work here!