fixed NodePickler not to choke on unhashable objects
fixed NodePickler not to choke on unhashable objects

armandobs14/rdflib,yingerj/rdflib,RDFLib/rdflib,yingerj/rdflib,armandobs14/rdflib,ssssam/rdflib,ssssam/rdflib,avorio/rdflib,dbs/rdflib,armandobs14/rdflib,yingerj/rdflib,marma/rdflib,avorio/rdflib,RDFLib/rdflib,RDFLib/rdflib,dbs/rdflib,marma/rdflib,dbs/rdflib,RDFLib/rdflib,avorio/rdflib,marma/rdflib,yingerj/rdflib,armandobs14/rdflib,ssssam/rdflib,avorio/rdflib,dbs/rdflib,ssssam/rdflib,marma/rdflib