If not explicitly configured the versioned_writes middleware
should be auto-inserted in the pipeline after slo and dlo, which
is where the versioned_writes filter section's comments say it
should be in proxy-server.conf-sample. At the moment it can end up
being placed ahead of slo and dlo if they have been explicitly
configured, which results in the linked bug manifesting.
Closes-Bug: #1537042
Change-Id: I6ac95a331f4ef0d4887311940acc6f8bc00fb4eb