Allow explicit number type declaration

[pcan]

I'm currently trying to send messages from amqp.node to a Java Spring Application, that expects an header as long, see this listener for example:

@RabbitListener(queues = "testQueue")
public void handleRequest(@Payload RequestBean bean, @Header("trace-id") long traceId) {
    // some logic here...
}

I'm getting a type mismatch exception when the handleRequest method gets called, since the expected type for trace-id is long (64-bit integer), but it arrives as byte (8-bit integer). I found that this behaviour is implemented here, and at the moment there is no way to force the number type from outside.

I think the codec may handle these special cases like it does for timestamp (see here and here), by adding 'int8', 'int16', 'int32' and 'int64' to that switch-case block, and let the user choose the right integer type if needed. The existing heuristic may not be changed, to avoid a breaking change.
So, I may write something like this:

var options = {
	headers: {
		'trace-id': {'!': 'int64', value: 12}
	},
	contentType: 'application/json',
	contentEncoding: 'UTF-8'
};

channel.publish('myExchange', 'routing.key', new Buffer(JSON.stringify(message)), options);

[squaremo]

I like this idea. You're right that it need not interfere with the existing encodings. I'm not sure there are signed 32 bit integers in AMQP 0.9.1 (or RabbitMQ's codec, anyway). But hence or otherwise, this would be fairly easy to add. PR? :)

[michaelklishin]

This is not a bad idea. Numerical type interop between languages is a pain and sometimes more control is the only way to achieve it.

On a related note, this is one of the reasons to avoid using message headers where possible and
stick to serialised message bodies.

[pcan]

Thanks. I will submit a PR asap.