Datepicker does not correctly disable the go back arror but does the go forward

[ryanbowden]

Bug description:

When using the date picker if you use the arrows to select a date in the past once you reach the minimum date of the date picker it does not disable. If you do the same but going forward the arrow correctly disables.

Link to minimally-working plunker that reproduces the issue:

You can see the behaviour on the examples site - https://ng-bootstrap.github.io/#/components/datepicker/examples

Just go to the minimum date and then go to the maximum

Version of Angular, ng-bootstrap, and Bootstrap:

We are using
ng-bootstrap: 1.0.0-beta.8
Bootstrap: 4.0.0-beta

The example site:
ng-bootstrap: 1.0.0-beta.9
Bootstrap: 4.0.0-beta.3