[Critical] Fix map[key]value, when key is uint64, it will overflow

[xtutu]

for example

package mongo_test_test

import (
    "encoding/json"
    "fmt"
    "github.com/globalsign/mgo/bson"
    "math"
    "testing"
)

func Stringify(o interface{}) string {
    data, err := json.Marshal(o)
    if err != nil {
        return err.Error()
    }
    return string(data)
}

type Data struct {
    Type int32             `bson:"Type"`
    Data map[uint64]string `bson:"Data"`
}

func TestBsonMap(t *testing.T) {
    mapData := make(map[uint64]string)

    numInt64 := uint64(math.MaxUint64)
    mapData[numInt64] = "a"
    data := &Data{
        Type: 100,
        Data: mapData,
    }
    byteData, err := bson.Marshal(data)
    if err != nil {
        fmt.Println(err)
    }

    var after = &Data{}
    err = bson.Unmarshal(byteData, after)
    fmt.Printf("%d\n", numInt64)
    fmt.Printf("%s\n", Stringify(after))
}

// the output should be:
// 18446744073709551615
// {"Type":100,"Data":{"18446744073709551615":"a"}}

// but got:
// 18446744073709551615
// {"Type":100,"Data":{"9223372036854775808":"a"}}