alert-dismissible being ignored by bootstrap-ui

[agarzon]

Hi.

I noticed that alert-dismissible class was not being working in combination wih friendsofcake/bootstrap-ui.

According the doc, the code should be like:

$event->getSubject()->params['class'] = 'alert alert-success alert-dismissible';

But that doesn't work... then I realized that if I pass the class as an array like:

$event->getSubject()->params['class'] = ['alert', 'alert-success', 'alert-dismissible'];

Then it works as should be.

So I guess is just matter of update the documentation or parse the string into array before passing it as params['class']

[ADmad]

Can you please submit a PR updating docs to use an array?