fix Page bug

hutool-db/src/main/java/cn/hutool/db/Page.java

@@ -134,10 +134,7 @@ public class Page implements Serializable {
 	 * @param orders 排序
 	 */
 	public void addOrder(Order... orders) {
-		if (null != this.orders) {
-			ArrayUtil.append(this.orders, orders);
-		}
-		this.orders = orders;
+		this.orders = ArrayUtil.append(this.orders, orders);
 	}
 	// ---------------------------------------------------------- Getters and Setters end
 

hutool-db/src/test/java/cn/hutool/db/PageTest.java

@@ -0,0 +1,17 @@
+package cn.hutool.db;
+
+import cn.hutool.db.sql.Order;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class PageTest {
+
+	@Test
+	public void addOrderTest() {
+		Page page = new Page();
+		page.addOrder(new Order("aaa"));
+		Assert.assertEquals(page.getOrders().length, 1);
+		page.addOrder(new Order("aaa"));
+		Assert.assertEquals(page.getOrders().length, 2);
+	}
+}