序列化结果是乱序的

[zhudusong]

protobuf.lua中_internal_serialize函数实现，ListFields方法遍历是无序的，会导致结果不稳定，在某些情景下无法反序列化。

local _internal_serialize = function(self, write_bytes)
    for field_descriptor, field_value in message_meta._member.ListFields(self) do
        field_descriptor._encoder(write_bytes, field_value)
    end
end

改动：根据field的key的index排序，再进行遍历。

local _internal_serialize = function(self, write_bytes)
    local field_list = {}
    for k, v in pairs(self._fields) do
        field_list[k.index + 1] = k
    end
    local n = table.maxn(field_list)
    for i = 1, n do
        local descriptor = field_list[i]
        if descriptor then
            value = self._fields[descriptor]
            if _IsPresent(descriptor, value) then
                descriptor._encoder(write_bytes, value)
            end
        end
    end
end