Don't overwrite existing clause builders

Author: jinzhu

sqlite.go

@@ -76,7 +76,9 @@ func (dialector Dialector) Initialize(db *gorm.DB) (err error) {
 	}
 
 	for k, v := range dialector.ClauseBuilders() {
-		db.ClauseBuilders[k] = v
+		if _, ok := db.ClauseBuilders[k]; !ok {
+			db.ClauseBuilders[k] = v
+		}
 	}
 	return
 }