[backport] Updating a blob with new data does not update SG's metadata on sync

Reported here: https://forums.couchbase.com/t/updating-blob-is-not-including-metadata/24012

and here: https://forums.couchbase.com/t/metadata-error-when-updating-attachments-with-a-file-already-used-previously/25364

It looks like creating a document with a blob, pushing it up to SG, and then updating that blob with new data that already exists in another doc does not send the updated metadata to Sync Gateway.

Need to try and repro using an Android Lite client (both reporters were using Android, which may provide a clue), and see whether it's a SG or Lite issue.