New attachment metadata not preserved on CAS retry during document update

Need to attempt to reproduce a loss of attachment data in the following scenario:

1. Document exists at some revision (3-a, say)
2. A new revision arrives (4-a) via the REST API, adding attachments to the document
3. Before that write completes, a write for revision 5-a arrives
4. Revision 4-a completes, adding attachment and attachment metadata to the document
5. The write for 5-a fails due to CAS, then retries
6. On second attempt, the write for 5-a succeeds, but _attachments metadata is being lost

This is only reported as happening in this particular CAS retry scenario. Should be feasible to write a unit test to try to reproduce this behaviour.