golang向interface{}解析json时把所有的数字都当成float64

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    var v map[string]interface{}
    jsonstr := `{"id":13,"name":"胖胖","weight":216.5,"dd":"123"}`
    json.Unmarshal([]byte(jsonstr), &v)
    for k, v1 := range v {
        fmt.Print(k, " = ")
        switch v1.(type) {
        case int:
            fmt.Println(v1, "is an int value.")
        case string:
            fmt.Println(v1, "is a string value.")
        case int64:
            fmt.Println(v1, "is an int64 value.")
        case float64:
            fmt.Println(v1, "is an float64 value.")
        default:
            fmt.Println(v1, "is an unknown type.")
        }
    }
}

API server listening at: 127.0.0.1:36104
weight = 216.5 is an float64 value.
dd = 123 is a string value.
id = 13 is an float64 value.
name = 胖胖 is a string value.
Process exiting with code: 0