OPS BibField: complement support for CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS

To be stashed with 9b94e59b7c926025507ccb163d3dec8578a0423c

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge pull request #142 from kaplun/bibfield-fix-api-for-hidden-fields

OPS BibField: complement support for CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS