Fix issue updating models with foreign key constraints (go-gorm#1988)

* fix update callback to not try to write zero values when field has default value

* fix to update callback for gorm tests

callback_update.go

@@ -76,7 +76,9 @@ func updateCallback(scope *Scope) {
 			for _, field := range scope.Fields() {
 				if scope.changeableField(field) {
 					if !field.IsPrimaryKey && field.IsNormal {
-						sqls = append(sqls, fmt.Sprintf("%v = %v", scope.Quote(field.DBName), scope.AddToVars(field.Field.Interface())))
+						if !field.IsForeignKey || !field.IsBlank || !field.HasDefaultValue {
+							sqls = append(sqls, fmt.Sprintf("%v = %v", scope.Quote(field.DBName), scope.AddToVars(field.Field.Interface())))
+						}
 					} else if relationship := field.Relationship; relationship != nil && relationship.Kind == "belongs_to" {
 						for _, foreignKey := range relationship.ForeignDBNames {
 							if foreignField, ok := scope.FieldByName(foreignKey); ok && !scope.changeableField(foreignField) {