SES-1803 : Reuse createBGP in createDataBody
SES-1803 : Explicitly return rdf:langString in EvaluationStrategyImpl

This should have been the expected behaviour for RDF-1.1 Language
Literals, but easier to check for it first to make sure.

Leaving xsd:string simple literal case that should never be hit as a
safe backup.
SES-1803 : QueryEvaluationUtil.isStringLiteral needs to return true for
language literals to fit SPARQL definition semantics