serializers: PID inclusion in marshmallow context
serializers: PID inclusion in marshmallow context

* Adds the 'pid' of the record that is being serialized in the
  marshmallow context, if not already there.

tiborsimko/invenio-records-rest,inveniosoftware/invenio-records-rest,inveniosoftware/invenio-records-rest,tiborsimko/invenio-records-rest