Timepicker does not recognize AM/PM change (Meridian)

[tilowestermann]

Bug description:

Using a timepicker with meridian set to true, I expect that entering "22" as the hour results in an update to "10" and a switch from "AM" to "PM" and vice versa. This does not happen in the current version.
Using the spinner to update the time, the switch from AM to PM (and vice versa) works.

Link to minimally-working plunker that reproduces the issue:

See demo page: https://ng-bootstrap.github.io/#/components/timepicker/examples

Version of Angular, ng-bootstrap, and Bootstrap:

Angular: 4.2.6

ng-bootstrap: 1.0.0-alpha.28

Bootstrap: 4.0.0-alpha.6