binding: support unix time (#1980)

* binding: support unix time

ref:#1979

* binding: support unix time

add test file
modify readme

```golang
package main

import (
        "fmt"
        "github.com/gin-gonic/gin"
        "time"
)

type shareTime struct {
        CreateTime time.Time `form:"createTime" time_format:"unixNano"`
        UnixTime   time.Time `form:"unixTime" time_format:"unix"`
}

func main() {
        r := gin.Default()
        unix := r.Group("/unix")

        testCT := time.Date(2019, 7, 6, 16, 0, 33, 123, time.Local)
        fmt.Printf("%d\n", testCT.UnixNano())

        testUT := time.Date(2019, 7, 6, 16, 0, 33, 0, time.Local)
        fmt.Printf("%d\n", testUT.Unix())

        unix.GET("/nano", func(c *gin.Context) {
                s := shareTime{}

                c.ShouldBindQuery(&s)

                if !testCT.Equal(s.CreateTime) {
                        c.String(500, "want %d got %d", testCT.UnixNano(), s.CreateTime)
                        return
                }

                c.JSON(200, s)
        })

        unix.GET("/sec", func(c *gin.Context) {
                s := shareTime{}

                c.ShouldBindQuery(&s)

                if !testUT.Equal(s.UnixTime) {
                        c.String(500, "want %d got %d", testCT.Unix(), s.UnixTime)
                        return
                }

                c.JSON(200, s)

        })

        r.Run()
}

```

* Contraction variable scope

binding/form_mapping.go

@@ -266,6 +266,24 @@ func setTimeField(val string, structField reflect.StructField, value reflect.Val
 		timeFormat = time.RFC3339
 	}
 
+	switch tf := strings.ToLower(timeFormat); tf {
+	case "unix", "unixnano":
+		tv, err := strconv.ParseInt(val, 10, 0)
+		if err != nil {
+			return err
+		}
+
+		d := time.Duration(1)
+		if tf == "unixnano" {
+			d = time.Second
+		}
+
+		t := time.Unix(tv/int64(d), tv%int64(d))
+		value.Set(reflect.ValueOf(t))
+		return nil
+
+	}
+
 	if val == "" {
 		value.Set(reflect.ValueOf(time.Time{}))
 		return nil