Log Message:
Update ruby-net-http-persistent to 2.5.2.

Changes:
  * The proxy may be changed at any time.
  * The allowed SSL version may now be set via #ssl_version.
    Issue #16 by astera
  * Added Net::HTTP::Persistent#override_headers which allows overriding
  * Net::HTTP default headers like User-Agent.  See
    Net::HTTP::Persistent@Headers for details.  Issue #17 by andkerosine
  * net-http-persistent now complains if OpenSSL::SSL::VERIFY_PEER is equal to
    OpenSSL::SSL::VERIFY_NONE.
  * Bug fixes.