Add 'omitempty' to args and kwargs

[jcelliott]

https://github.com/tavendo/WAMP/blob/master/spec/basic.md#empty-arguments-and-keyword-arguments

[beatgammit]

Perhaps optional fields can have a wamp tag:

type Call struct {
    Request ID
    Options map[string]interface{}
    Procedure URI
    Arguments []interface{} `wamp:"omitempty"`
    ArgumentsKw map[string]interface{} `wamp:"omitempty"`
}

Fields will be traversed backwards looking for omitempty, and those that are nil or 0-length are omitted.