patch for issue 456

Do not filter out duplicate subscriptions on the XSUB side of
XSUB/XPUB, so that ZMQ_XPUB_VERBOSE doesn't get blocked by forwarding
devices (as long as the devices all use ZMQ_XPUB_VERBOSE)
Code formatting
#456 was not in 3.2.2 but will be in 3.2.4