Allow Net::HTTP to disable SSLv3 via SSLContext

Net::HTTP does not have a way to customize SSLContext#options. This
patch allows users to disable SSLv3 via TLS: Thread Local Storage.

Bear in mind that you need to declare this by yourself.
  Thread.current[:SET_SSL_OP_NO_SSLv3] = true

"!SSLv3" in :ciphers disables TLSv1, etc., too so this should be the
only way to disable SSLv3 for Net::HTTP.