yahns 1.8.0 - minor updates

Most notably, the Rack response body is now closed during rack.hijack.

  Middlewares such as Rack::Lock (used by Rails) break badly unless
  the response body is closed on hijack, so we will close it to follow
  the lead of other popular Rack servers.

  While it's unclear if there's anybody using rack.hijack besides
  yahns/proxy_pass we'll try to emulate the behavior of other servers
  as much as possible.

  ref: https://github.com/ngauthier/tubesock/issues/10

We'll also support SIGWINCH if not daemonized

  This has no effect for the (default) single process case with
  no master/worker relationship as that does not support SIGWINCH.

 Some process managers such as foreman and daemontools rely on
 yahnsnot daemonizing, but we still want to be able to process
 SIGWINCH in that case.

 stdout and stderr may be redirected to a pipe (for cronolog or
 similar process), so those are less likely to be attached to a TTY
 than stdin.  This also allows users to process SIGWINCH when running
 inside a regular terminal if they redirect stdin to /dev/null.

 This follows unicorn commit a6077391bb62d0b13016084b0eea36b987afe8f0
 Thanks to Dan Moore for suggesting it on the unicorn list.

A few more minor changes, more memory reduction changes coming...

* proxy_pass: no point in closing StringIO
* proxy_pass: allow filtering or overriding response headers
* support SIGWINCH even if not daemonized
* use Unicorn::HttpParser#response_start_sent accessor
* reduce inline constant cache overheads
* proxy_pass: skip tests if kcar is missing
* ensure body is closed during hijack