containerClass is not passed if the tooltip isOpen is true

[probfacile]

Bug description or feature request:

If you start a tooltip as open (isOpen="true") when it is displayed the first time will not have a class. when it hides, and reopen it will have the correct class.

Plunker/StackBlitz that reproduces the issue:

https://stackblitz.com/edit/angular-ngxbootstrapmodal?file=app%2Fapp.component.html
just wait page loads the go with mouse over the button it will hide and when you reopen it will have correct class

Versions of ngx-bootstrap, Angular, and Bootstrap:

check the stackblitz dependencies. tested 2.0.2 and 2.0.4

ngx-bootstrap: 2.0.4
Angular: 5.0.9
Bootstrap: 3.3.7

Build system: Angular CLI, System.js, webpack, starter seed:

check the stackblitz