add json ASCII string render (#1358)

add a json render that rendering json as ASCII string

README.md

@@ -900,6 +900,29 @@ func main() {
 }
 ```
 
+#### AsciiJSON
+
+Using AsciiJSON to Generates ASCII-only JSON with escaped non-ASCII chracters.
+
+```go
+func main() {
+	r := gin.Default()
+
+	r.GET("/someJSON", func(c *gin.Context) {
+		data := map[string]interface{}{
+			"lang": "GO语言",
+			"tag":  "<br>",
+		}
+
+		// will output : {"lang":"GO\u8bed\u8a00","tag":"\u003cbr\u003e"}
+		c.AsciiJSON(http.StatusOK, data)
+	})
+
+	// Listen and serve on 0.0.0.0:8080
+	r.Run(":8080")
+}
+```
+
 ### Serving static files
 
 ```go