Update exporters.py

Hi,

Thanks for this, work well with me. Just had to fix the below to get it to work but not 100% sure why this is required since item[field_name] is already a string ?

value_list.append(self.serialize_field(field, field_name, (item[field_name])))

value_list.append(self.serialize_field(field, field_name, str(item[field_name])))

Gilles