binding: support map for map form

[thinkerou]

Please @javierprovecho @appleboy review the pull request, thanks a lot!

ref: #1362 #398

Idea: declare one in map[string]interface{} (MUST), and user inputs -F 'in={"i":1}'.

Support map for map form, can use the curl command:

curl -v http://localhost:8080/loginForm -F 'user={"a":"hello","b":2, "c": 3.14}' -F 'password=123'

and the following golang code:

package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

// Binding from JSON
type Login struct {
	User     map[string]interface{} `form:"user"`
	Password string                 `form:"password"`
}

func main() {
	router := gin.Default()

	router.POST("/loginForm", func(c *gin.Context) {
		var form Login
		// This will infer what binder to use depending on the content-type header.
		if err := c.ShouldBind(&form); err == nil {
			if form.User["a"].(string) == "hello" && form.User["b"].(float64) == 2 && form.User["c"].(float64) == 3.14 && form.Password == "123" {
				c.JSON(http.StatusOK, gin.H{"status": "you are logged in"})
			} else {
				c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
			}
		}
	})

	// Listen and serve on 0.0.0.0:8080
	router.Run(":8080")
}

it will output the result:

*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> POST /loginForm HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Length: 269
> Expect: 100-continue
> Content-Type: multipart/form-data; boundary=------------------------02819574d8de733f
>
< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
< Content-Type: application/json; charset=utf-8
< Date: Sun, 10 Jun 2018 05:05:13 GMT
< Content-Length: 30
<
* Connection #0 to host localhost left intact
{"status":"you are logged in"}

[codecov]

Codecov Report

Merging #1387 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1387      +/-   ##
==========================================
+ Coverage   98.48%   98.49%   +<.01%     
==========================================
  Files          41       41              
  Lines        2046     2057      +11     
==========================================
+ Hits         2015     2026      +11     
  Misses         19       19              
  Partials       12       12

Impacted Files	Coverage Δ
binding/form_mapping.go	100% <100%> (ø)	⬆️

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[vkd]

I think better to use this ptr to real map. It's give you the opportunity to unmarshal from any types of map.

Suggested change

	err := json.Unmarshal([]byte(inputValue[0]), &m)
	err := json.Unmarshal([]byte(inputValue[0]), val.Field(i).Addr().Interface())

[thinkerou]

@appleboy please help me review the pr, thanks!

[vkd]

I have already added support of map in binding and these tests in my PR #1749