Update ruby-net-http-persistent to 2.8.

=== 2.8 / 2012-10-17

* Minor enhancements
  * Added Net::HTTP::Persistent::detect_idle_timeout which can be used to
    determine the idle timeout for a host.
  * The read timeout may now be updated for every request.  Issue #33 by
    Mislav Marohnić
  * Added NO_PROXY support.  Pull Request #31 by Laurence Rowe.
  * Added #cert and #key aliases for Net::HTTP compatibility.  Pull request
    #26 by dlee.
  * The artifice gem now disables SSL session reuse to prevent breakage of
    testing frameworks.  Pull Request #29 by Christopher Cooke.
  * Disabled Net::HTTP::Persistent::SSLReuse on Ruby 2+.  This feature is now
    built-in to Net::HTTP.
* Bug fixes
  * Socket options are set again following connection reset.  Pull request #28
    by cmaion.
  * #shutdown now works even if no connections were made.  Pull Request #24 by
    James Tucker.
  * Updated test RSA key size to 1024 bits.  Bug #25 by Gunnar Wolf.
  * The correct host:port are shown in the exception when a proxy connection
    fails.  Bug #30 by glebtv.

(taca)