Add option to strip undefined values off object as JSON does

[willium]

In JSON, undefined values stripped off the object

> JSON.parse(JSON.stringify({a: undefined}))
{}
> JSON.parse(JSON.stringify({a: undefined})).a
undefined

and undefined values in arrays are converted to null

It would be great if there were some option that removed undefined props at encode time.

[gfx]

It sounds good to reduce the size of payloads, but it will consume more time in encoding because MessagePack's map type has a "size" data before its contents, so the encoder must count the number of entries to encode. So I'll add it but won't make it the default.