fix Button bug in 4.6.0

examples/routers/button.vue

@@ -309,9 +309,11 @@
             </ButtonGroup>
         </div>
         <Divider></Divider>
-        <Button to="http://baidu.com" replace>http跳转</Button>
+        <Button to="http://baidu.com">http跳转</Button>
+        <Button to="http://baidu.com" target="_blank">http跳转(_blank)</Button>
         <Button to="https://iviewui.com" replace>https跳转</Button>
         <Button :to="{path: '/icon'}">Router 对象跳转</Button>
+        <Button :to="{path: '/icon'}" target="_blank">Router 对象跳转(_blank)</Button>
     </div>
 </template>
 <script>

src/mixins/link.js

@@ -50,6 +50,7 @@ export default {
                 const route = router.resolve(this.to, current, this.append);
                 to = route ? route.href : this.to;
             }
+            if (typeof this.to === 'string') return; // 会跳转两次
             window.open(to);
         },
         handleClick (new_window = false) {