Add an option to support predicable encoding

[hh9527]

// file: lib/encoder.js:201 function encodeObject
    for (key in obj) {
      if (obj.hasOwnProperty(key) &&
        obj[key] !== undefined &&
        typeof obj[key] !== 'function') {
        ++length
        acc.push(encode(key, true))
        acc.push(encode(obj[key], true))
      }
    }

this will cause the properties order is unpredicable, that will cause some digest mechanism will failed: same value in different case produce different encoded result.

[mcollina]

It follows the same order as JSON.

[hh9527]

Yes, it follows the order from JSON. but JSON's order is unpredicable. so MsgPack is also unpredicable.

[mcollina]

Send a PR AND i will review it.