records: lazy type introduction in MarcXML export

* Checks if the string is a lazy string and converts it into a list in
  order not to be split in letters while exporting to MarcXML.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>