response size limiter

响应长度限制中间件，可以限制响应数据的长度，避免返回过大的数据导致网络占用过大。此中间件主要用于避免一些非法调用等导致查询过多数据。

Example

package main
import (
    "bytes"
    "time"
    "github.com/vicanso/elton"
    "github.com/vicanso/elton/middleware"
)
func main() {
    e := elton.New()
    e.Use(middleware.NewResponseSizeLimiter(middleware.ResponseSizeLimiterConfig{
        // 1MB
        MaxSize: 1024 * 1024,
    }))
    e.GET("/users/me", func(c *elton.Context) (err error) {
        time.Sleep(time.Second)
        c.BodyBuffer = bytes.NewBufferString(`{
            "account": "tree",
            "name": "tree.xie"
        }`)
        return nil
    })
    err := e.ListenAndServe(":3000")
    if err != nil {
        panic(err)
    }
}