The `--id-field` option works differently for Catmandu::Importer::Solr as for Catmandu::Store::Solr

[phochste]

When doing:

$ catmandu convert Solr --url http://localhost:8984/solr --id_field myfield

I get records where the content of myfield is put into the _id field of the resulting records:

---
_id: archive.ugent.be:00003832-ADE7-11E0-92CA-80A237D8FA8C
accessRights:
- ugent
date: 2016-08-26T08:55:51.45Z

When doing:

$ catmandu export Solr --url http://localhost:8984/solr --id_field myfield

the _id has an undefined content:

---
_id: ~
accessRights:
- ugent
date: 2016-08-26T08:55:51.45Z