models: update mySQL charset=utf8mb4 (#4880)

* Add utf8mb4 to connStr for mysql. fix #4131

* Update models.go. charset=utf8mb4 for mysql.

charset=utf8mb4 for mysql.

models/models.go

@@ -134,10 +134,10 @@ func getEngine() (*xorm.Engine, error) {
 	switch DbCfg.Type {
 	case "mysql":
 		if DbCfg.Host[0] == '/' { // looks like a unix socket
-			connStr = fmt.Sprintf("%s:%s@unix(%s)/%s%scharset=utf8mb4,utf8&parseTime=true",
+			connStr = fmt.Sprintf("%s:%s@unix(%s)/%s%scharset=utf8mb4&parseTime=true",
 				DbCfg.User, DbCfg.Passwd, DbCfg.Host, DbCfg.Name, Param)
 		} else {
-			connStr = fmt.Sprintf("%s:%s@tcp(%s)/%s%scharset=utf8mb4,utf8&parseTime=true",
+			connStr = fmt.Sprintf("%s:%s@tcp(%s)/%s%scharset=utf8mb4&parseTime=true",
 				DbCfg.User, DbCfg.Passwd, DbCfg.Host, DbCfg.Name, Param)
 		}
 	case "postgres":