test triples_with_o, triples_with_po and triples_with_so with explicit xsd:string datatype

[KonradHoeffner]

The test file only contains literals that either have a different datatypes or none at all datatype, so we throw away the implicit xsd:string to match correctly. However explicit xsd:string needs to be supported as well.

[KonradHoeffner]

Tested in a locally modified version, it works.