fix(Countdown): 修复倒计时不生效的问题

close #1464

Author: smileShirmy

examples/pages/components/view/pages/countdown/index.wxml

@@ -3,7 +3,7 @@
   <content-title name="Countdown" describe="倒计时">
     <content-card l-content="content" name="基本倒计时">
       <l-countdown />
-      <l-countdown time="2020-5-1 18:30"/>
+      <l-countdown time="2022-5-1 18:30"/>
     </content-card>
 
     <content-card l-content="content" name="传入时间为秒">

src/behaviors/countdown.js

@@ -3,7 +3,7 @@ export default Behavior({
   behaviors: [],
   properties: {
     time: {
-      type: Object,
+      optionalTypes: [String, Object],
       value: new Date().getTime() + 86400000,
       observer: function (newVal, oldVal) {
         if (newVal && !oldVal) {