_removed entry not sent with delta sync

The scenario is as follows:

1. Start a SG instance with one user (autotest) assigned access to a particular channel (ABC)
2. Create several documents on Couchbase Lite that get routed to the ABC channel
3. Modify all the documents via SG HTTP so that they get routed to another channel instead (e.g. "unknown" channel)
4. Run a pull replication on CBL

With delta sync disabled this results in an exchange that finally ends in the following BLIP message for each revision:

Frame Flags: Compressed|NoReply (0x28)

Properties Length: 153

Properties: history:1-f93627c3c7e953ef8d319b20cd7741cf9dad1c3e:Content-Type:application/json:Profile:rev:id:cbl_0:rev:2-8619ff078f2b2751592e4ffd45f262b7:sequence:12

Message Body: {"_removed":true}

However, with delta sync enabled the following gets sent instead:

Frame Flags: Compressed|NoReply (0x28)

Properties Length: 205

Properties: Content-Type:application/json:Profile:rev:id:cbl_0:rev:2-58f62bd7e692d3f482ed9a9f9a775d48:sequence:12:history:1-e1d6c42d605810a2eb07da07ba3b022c326fb292:deltaSrc:1-e1d6c42d605810a2eb07da07ba3b022c326fb292

Message Body: {"channels":{"0":"unknown"}}

This results in the access removed event never getting triggered.