Deleting statements in a context

[kbogtob]

With the RDF::Repository, I can set a context with the #set_context method. After than, every statement I'm going to insert into the repository will be assigned this context.

But when I delete statements from the repository, the context that is used to delete the statements is the one already assigned to the statement (RDF::Statement#context= or #graph=). So the two operations are not symmetric at all.

I'm going to do a PR in a few to adjust that behavior.

[kbogtob]

PR done, waiting to be merged #12