feat: add SecureJSON func to prevent json hijacking

[easonlin404]

Implement #433, SecureJSON default prepends "while(1)," to response body if the given struct is array values. Examples are shown below:

package main

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

)
func main() {
	r := gin.Default()
	// r.SecureJsonPrefix(")]}',\n")  // Default "while(1);"

	r.GET("/someJSON", func(c *gin.Context) {
		fruit := []string{"apple", "peach", "pear"}

		// Will output  :   while(1);["apple","peach","pear"]
		c.SecureJSON(http.StatusOK, fruit)
	})
	
	r.Run(":8080")
}

Please review and advise, thanks.

[codecov]

Codecov Report

Merging #987 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #987      +/-   ##
==========================================
+ Coverage   96.51%   96.52%   +0.01%     
==========================================
  Files          16       16              
  Lines        1406     1412       +6     
==========================================
+ Hits         1357     1363       +6     
  Misses         39       39              
  Partials       10       10

Impacted Files	Coverage Δ
gin.go	91.74% <100%> (+0.16%)	⬆️
context.go	96.31% <100%> (+0.02%)	⬆️

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[javierprovecho]

@easonlin404 thanks! 😄