yahns 1.12.3 - more fixes and doc updates

This release only contains two changes since v1.12.2:

* proxy_http_response: workaround non-terminated backends

  The first is a fix for the undocumented and unstable "proxy_pass"
  feature:
  https://yhbt.net/yahns-public/20160406062556.10988-1-e@80x24.org/t/

  However, I'm considering supporting proxy_pass as a stable API:
  https://yhbt.net/yahns-public/20160220081619.GA10850@dcvr.yhbt.net/t/

* doc: recommend "verify_mode: OpenSSL::SSL::VERIFY_NONE"

  Only a documentation change prompted by the discovery that
  some browsers/platforms will try to prompt users for client
  certs:
  https://yhbt.net/yahns-public/20160316003434.GA14791@dcvr.yhbt.net/t/