fix #2234

1. 修复small size下沉1px的问题
2. 修复左右阴影被截断的问题
3. 修复有值时hover及focus样式丢失的问题

components/cascader/style/index.less

@@ -9,14 +9,14 @@
     display: block;
     cursor: pointer;
     width: 100%;
+    background: rgba(255, 255, 255, 0);
+    z-index: 1;
   }
   &-picker {
     position: relative;
     display: inline-block;
     cursor: pointer;
-    vertical-align: middle;
     font-size: @font-size-base;
-    overflow: hidden;
 
     &-disabled {
       cursor: not-allowed;