basic auth: fix timing oracle (#2609)

Co-authored-by: thinkerou <thinkerou@gmail.com>

auth.go

@@ -5,6 +5,7 @@
 package gin
 
 import (
+	"crypto/subtle"
 	"encoding/base64"
 	"net/http"
 	"strconv"
@@ -30,7 +31,7 @@ func (a authPairs) searchCredential(authValue string) (string, bool) {
 		return "", false
 	}
 	for _, pair := range a {
-		if pair.value == authValue {
+		if subtle.ConstantTimeCompare([]byte(pair.value), []byte(authValue)) == 1 {
 			return pair.user, true
 		}
 	}