Review Lang mappings between JSON and JSONLD when the expected content is RDF

[enridaga]

In the SPARQL Anything CLI we want to support files with extension .json to be loaded as RDF with the -l option (See #339)
However, the mapping needs to be corrected when we rely on it loading files from the web from within the fx engine, see RDFTriplifier.getLang.
Although things work fine, this is a workaround and we should review this approach.