Fix empty table placehoder style

[yesmeck]

1. Fixed header 的时候"暂无数据"会被遮住；
2. small 的时候下面会有两条边框。

http://codepen.io/yesmeck/pen/VPqqZR?editors=0010

Before:

After:

[mention-bot]

@yesmeck, thanks for your PR! By analyzing the history of the files in this pull request, we identified @afc163, @megawac and @ddcat1115 to be potential reviewers.

[coveralls]

Coverage remained the same at 82.818% when pulling 972df1a on fix-empty-table-style into 1490a40 on master.

[afc163]

给 .ant-table-placeholder 加一个 position: relative; 即可。

[yesmeck]

调整了，顺便把边框也调整了下。

[coveralls]

Coverage remained the same at 82.3% when pulling a8e8bfa on fix-empty-table-style into 597e020 on master.

[coljung]

Out of curiosity, is there a way to modify the placeholder that results in the table when there is no data?

Currently we get this attribute:

<div class="ant-table-placeholder">No Data</div>

I thought there was a way to customize it, but could not find how.

[yesmeck]

@coljung See #3822

[coljung]

thx!