Fix get_field_value

[moshthepitt]

get_field_value will throw an error is the field is not hashable. e.g. for JSON field

Suggested fix:

    def get_field_value(self, field):
        """Get the value of a field."""
        if field.is_relation:
            try:
                return str(getattr(self.object, field.name))
            except AttributeError:
                return None
        try:
            # pylint: disable=protected-access
            return self.object._get_FIELD_display(field)
        except TypeError:
            # for some reason this is not hashable
            field_value = getattr(self.object, field.attname)
            return json.dumps(field_value, cls=DjangoJSONEncoder)