Axis-common hover label border default color doesn't respond correctly to light bgcolor

<img width="258" alt="screen shot 2018-04-14 at 11 26 15 am" src="https://user-images.githubusercontent.com/2678795/38769798-b392be5a-3fd6-11e8-822f-875630461dc8.png">
where I've used:

```js
layout: {hoverlabel: {bgcolor: 'white', font: {color: '#444'}}}
```

the label at the bottom (Sep 12, 2016) should have a dark border just like the main hover label.