http subrequest() method.

http subrequest() method.

Creates an nginx's subrequest with the specified arguments and
registers a finalization callback.

req.subrequest(<uri>[, <options>[, <callback>]]):
    uri - string.
    options - string | object.
        string value - uri arguments.
        object value can contain:
            args, body, method all are string values.
    callback - function with the following argument:
        reply - the result object with the following properties:
            uri, method, status, contentType, contentLength,
            headers, args, body, parent.