yahns 1.12.2 - minor doc and TLS fixes

This release ensures OpenSSL::SSL::SSLContext#session_id_context
is always set for OpenSSL users.  It won't overwrite existing
settings, but setting it to a random value is necessary to
ensure clients do not get aborted connections when attempting to
use a session cache.

No need to actually upgrade if you're on 1.12.1, you may add the
following to your yahns_config(5) file where
OpenSSL::SSL::SSLContext is configured:

    # recommended, not required.  This sets safer defaults
    # provided by Ruby on top of what OpenSSL gives:
    ssl_ctx.set_params

    # required, and done by default in v1.12.2:
    ssl_ctx.session_id_context ||= OpenSSL::Random.random_bytes(32)

yahns gives you full control of of how OpenSSL::SSL::SSLContext is
configured.  To avoid bugs, yahns only ensures
OpenSSL::SSL::SSLContext#session_id_context is set (if not previously
set by the user) and calls OpenSSL::SSL::SSLContext#setup before
spawning threads to avoid race conditions.  yahns itself does not and
will not enforce any opinion on the compatibility/performance/security
trade-offs regarding TLS configuration.

Note: keep in mind using an SSL session cache may be less useful
with yahns because HTTP/1.1 persistent connections may live
forever :)

3 bug/doc fixes on top of v1.12.1:
      document OpenSSL::SSL::SSLContext#set_params use
      ssl: ensure is session_id_context is always set
      test/*: fix mktmpdir usage for 1.9.3