c.ShouldBindBodyWith should not consume a request body

[momotaro98]

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Description

This issue is related to #439, #1810.

I think c.ShouldBindBodyWith should not consume a request body for next binding.

When I read the current README doc about this method, I thought it doesn't but it does so I've got an EOF error like the example below.

I think it's now confusing for users.

Thanks.

How to reproduce

package main

import (
	"encoding/json"
	"fmt"

	"github.com/gin-gonic/gin"
	"github.com/gin-gonic/gin/binding"
)

type MyStruct struct {
	Key string `json:"key" binding:"required"`
}

func MyMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		var h MyStruct

		if err := c.ShouldBindBodyWith(&h, binding.JSON); err != nil {
			c.JSON(400, gin.H{
				"message": fmt.Sprintf("%+v", err),
			})
			return
		}

		c.Next()
	}
}

func main() {
	r := gin.Default()
	r.Use(MyMiddleware())
	r.POST("/post", func(c *gin.Context) {
		var h MyStruct

		decoder := json.NewDecoder(c.Request.Body)
		err := decoder.Decode(&h)
		if err != nil {
				c.JSON(400, gin.H{
					"message": fmt.Sprintf("%+v", err),
				})
				return
		}

		c.JSON(200, gin.H{
			"message": "ok",
		})
	})
	r.Run() // listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
}

Expectations

$ curl --location --request POST 'http://localhost:8080/post' \
--header 'Content-Type: application/json' \
--data-raw '{
    "key": "value"
}'
{"message":"ok"}

Actual result

$ curl --location --request POST 'http://localhost:8080/post' \
--header 'Content-Type: application/json' \
--data-raw '{
    "key": "value"
}'
{"message":"EOF"}

Environment

• go version: go1.15 darwin/amd64
• gin version (or commit ref): 3100b7c
• operating system: darwin/amd64