Default placement not working

I have the following config:

```js
import VueTooltip from 'v-tooltip'
Vue.use(VueTooltip, {
defaultPlacement: 'bottom',
defaultClass: '',
defaultTemplate: `

`,
defaultArrowSelector: '.Tooltip-arrow',
defaultInnerSelector: '.Tooltip-body',
defaultTargetClass: 'has-tooltip',
defaultLoadingClass: 'is-loading',
defaultLoadingContent: '',
});
```

And HTML:

```html
warning
```

This fails to produce a tooltip.

I have to explicitly specify the placement in order for it to work:

```html
warning
```

Am I doing something wrong or is this a bug? I would expect that it would honour the configured default placement if I don't specify it.