Allow to enable SO_LINGER option with zero timeout

According to http://docs.oracle.com/javase/8/docs/api/java/net/StandardSocketOptions.html#SO_LINGER linger option is disabled by default. Currently if SocketConfig.soLinger is set to zero (default value is -1), then Socket.setSoLinger(false, 0) will be called, so linger will be disabled, which is undesirable. My suggestion is interpret zero soLinger value as Socket.setSoLinger(true, 0).

I understand that this change will break backward compatibility of config interpretation, but can we have this in 4.4?