When last_modified is passed as ETag for safety, sent headers are wrong

[leplatrem]

Using listRecords() the obtained last_modified for the collection is quoted.

If I pass the obtained value in the last_modified option on other methods, the sent header is quoted twice ('""12345""')

See https://github.com/Kinto/kinto-http.js/blob/v1.0.0/src/requests.js#L17

Note: another approach could be to unquote the last_modified value in the listRecords() result