[Popover] popover is not closed on contextmenu event if it's DOM element is not button

[eLeontev]

Type: defect;
version ngbBootstrap: latest
Browser to reproduce: Chrome (did not checked on other)
Preconditions:

1. create multiple popovers with custom triggers on contextmenu event (right click action)
2. set at least one of the DOM element with popover like DIV (any !== Button )

Steps to reproduce: right clicks between elements with popovers

Actual result: displayed popover is closed only on right click to outside of it only if its element is Button
Expected result: popover should always closes on right click outside. No matter its element type.

Example: https://stackblitz.com/edit/angular-emylps