Fix #784 set mysql datetime's type to NOT NULL

dialect_mysql.go

@@ -61,7 +61,11 @@ func (mysql) DataTypeOf(field *StructField) string {
 			}
 		case reflect.Struct:
 			if _, ok := dataValue.Interface().(time.Time); ok {
-				sqlType = "timestamp NULL"
+				if _, ok := field.TagSettings["NOT NULL"]; ok {
+					sqlType = "timestamp"
+				} else {
+					sqlType = "timestamp NULL"
+				}
 			}
 		default:
 			if _, ok := dataValue.Interface().([]byte); ok {

structs_test.go

@@ -42,7 +42,7 @@ type CreditCard struct {
 	ID        int8
 	Number    string
 	UserId    sql.NullInt64
-	CreatedAt time.Time
+	CreatedAt time.Time `sql:"not null"`
 	UpdatedAt time.Time
 	DeletedAt *time.Time
 }