Directive string support: Handle update case

[kkuegler]

The tippy directive properly supports string values on first render. However, when the component is updated, there was no string handling and the value was assumed to be an object.

Now we apply the same handling for the update case, too.

It would otherwise fail with something like this:

Error: [tippy]: `0` is not a valid option
    validateOptions
    validateOptions
    set
    componentUpdated
    ...