feat(form): Support default values for collections in form binding (#4048)

docs/doc.md

@@ -26,6 +26,7 @@
   - [Custom Validators](#custom-validators)
   - [Only Bind Query String](#only-bind-query-string)
   - [Bind Query String or Post Data](#bind-query-string-or-post-data)
+  - [Bind default value if none provided](#bind-default-value-if-none-provided)
   - [Collection format for arrays](#collection-format-for-arrays)
   - [Bind Uri](#bind-uri)
   - [Bind custom unmarshaler](#bind-custom-unmarshaler)
@@ -862,6 +863,53 @@ Test it with:
 curl -X GET "localhost:8085/testing?name=appleboy&address=xyz&birthday=1992-03-15&createTime=1562400033000000123&unixTime=1562400033"
 ```
 
+
+### Bind default value if none provided
+
+If the server should bind a default value to a field when the client does not provide one, specify the default value using the `default` key within the `form` tag:
+
+```
+package main
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+type Person struct {
+	Name      string    `form:"name,default=William"`
+	Age       int       `form:"age,default=10"`
+	Friends   []string  `form:"friends,default=Will;Bill"`
+	Addresses [2]string `form:"addresses,default=foo bar" collection_format:"ssv"`
+	LapTimes  []int     `form:"lap_times,default=1;2;3" collection_format:"csv"`
+}
+
+func main() {
+	g := gin.Default()
+	g.POST("/person", func(c *gin.Context) {
+		var req Person
+		if err := c.ShouldBindQuery(&req); err != nil {
+			c.JSON(http.StatusBadRequest, err)
+			return
+		}
+		c.JSON(http.StatusOK, req)
+	})
+	_ = g.Run("localhost:8080")
+}
+```
+
+```
+curl -X POST http://localhost:8080/person
+{"Name":"William","Age":10,"Friends":["Will","Bill"],"Colors":["red","blue"],"LapTimes":[1,2,3]}
+```
+
+NOTE: For default [collection values](#collection-format-for-arrays), the following rules apply:
+- Since commas are used to delimit tag options, they are not supported within a default value and will result in undefined behavior
+- For the collection formats "multi" and "csv", a semicolon should be used in place of a comma to delimited default values
+- Since semicolons are used to delimit default values for "multi" and "csv", they are not supported within a default value for "multi" and "csv"
+
+
 #### Collection format for arrays
 
 | Format          | Description                                               | Example                 |