add As function for gdb.Model; improve string quote handling for gdb

.example/database/gdb/mysql/gdb_value.go

@@ -2,23 +2,16 @@ package main
 
 import (
 	"github.com/gogf/gf/frame/g"
-	"github.com/gogf/gf/os/gtime"
 )
 
 func main() {
 	db := g.DB()
-	//db.SetDebug(true)
+	db.SetDebug(true)
 
-	type User struct {
-		Id   int
-		Name *gtime.Time
-	}
-
-	user := new(User)
-	e := db.Table("test").Where("id", 10000).Struct(user)
+	one, e := db.Table("order.order o").LeftJoin("user.user u", "o.uid=u.id").Where("u.id", 1).One()
 	if e != nil {
 		panic(e)
 	}
-	g.Dump(user)
+	g.Dump(one)
 
 }