fix(ArcPopup): 修复 direction 属性设置无效

close #1498

Author: smileShirmy

src/arc-popup/index.wxml

@@ -1,6 +1,6 @@
 <l-popup 
   show="{{show}}" 
-  direction="{{direction}}" 
+  contentAlign="{{direction}}" 
   transition="{{transition}}" 
   opacity="{{opacity}}" 
   locked="{{locked}}"