Do not alter the tooltip content on 'show' if it has already been rendered

- Create a tooltip
- Modify the tooltip content
- Hide it with `$el.tooltip('hide')`
- Show it again (`$el.tooltip('show')`)

The content of the tooltip is overwritten by the original content. I would be expecting this behavior if i had called `$el.tooltip('destroy')`, but not when calling 'hide'.

This pull request set the `.tooltip-inner` content only if it hasn't already been set which allow editing the tooltip content and not loosing the changes.

I need this because my tooltip is a bunch of `<i class="icon">` and i would like to toogle the `active` classes of theses icon and have those classes persist a `tooltip('hide')`/`tooltip('show')`.


Thanks!
Arthur