It is not possible to search for Literals in the graphdb backend

[francescalb]

The following fails:

list(ts.triples(object=Literal('a')))

This is the error message: QueryBadFormed: QueryBadFormed: A bad request has been sent to the endpoint: probably the SPARQL query is badly formed.

This is the code that poluates the TS:

from tripper import Triplestore, Literal

ts = Triplestore(backend="sparqlwrapper", base_iri=...., update_iri=...)

ts.add_triples([("http://www.example.org/test333", "http://www.example.org/test3", Literal("a"))])