Apart from DEFAULT_VERSIONS unfortunateness, overall it looks good and was tested to work.

For the record, I observe this problem on unpatched 12.0 with nginx-tests,
which manifests itself as a stalled transmit since select() never returns
ready read event, although data was sent and should be available on socket.
In tcpdump it's seen as after some time, client's TCP ACK sequence number
starts to never increment (stuck at 2701889 in the table), then server
performs retransmits and so on, until connection timeout.
Notably, this is reproducible on loopback with low SO_SNDBUF set on server.

I see no objection to commit the change in its current form.
See minor notes.

minor mdoc