fix(Grid): 修复 l-grid-item 样式改动导致 grid 组件不能居中

close #1052

Author: murongg

src/grid/index.less

@@ -5,6 +5,7 @@
 
     .l-grid-item {
         display: flex;
+        justify-content: center;
         flex-direction: column;
         text-align: center;
         box-sizing: border-box;