Fix multipart messages for xpub/xsub

[BryceCicada]

The following code snippet demonstrates that proxy is only forwarding the first part of a multipart message:

let zmq = require('zmq');
let subs = zmq.socket('xpub');
let pubs = zmq.socket('xsub');

subs.bindSync('tcp://127.0.0.1:3000');
pubs.bindSync('tcp://127.0.0.1:3001');
console.log('Proxy bound to to ports 3000 and 3001');
zmq.proxy(subs, pubs);


let subscriber = zmq.socket('sub');
subscriber.connect('tcp://127.0.0.1:3000');
subscriber.subscribe('');
console.log('Subscriber connected to port 3000');
subscriber.on('message', function (part1, part2) {
    console.log(part1.toString(), ':', part2 && part2.toString());
});


let publisher = zmq.socket('pub');
publisher.connect('tcp://127.0.0.1:3001');
console.log('Publisher connected to port 3001');
setInterval(() => {
    publisher.send(['foo', 'bar']);
}, 500);

wherein the publisher sends two parts, but the subscriber only receives the first part.

This change fixes the proxy so that it sends all parts to the subscriber.

Related issue #412

[reqshark]

I think it looks good, can we do:

[].slice.call(arguments)

or do we have to use the Array.prototype syntax?

[BryceCicada]

@reqshark Syntax changed as requested.

[reqshark]

thanks @BryceCicada!

[ronkorving]

LGTM, thanks!
@reqshark welcome back, it's been a while ;)