Dropup does not work in IE11 / BS4

[brgaulin]

Bug description or feature request:

Dropdowns in the up direction only work once in IE11. If you click a second time, they go in the down direction. (but still add the dropup class to the parent)

Plunker/StackBlitz that reproduces the issue:

The demo is affected: https://valor-software.com/ngx-bootstrap/#/dropdowns#dropup

Versions of ngx-bootstrap, Angular, and Bootstrap:

ngx-bootstrap: Tried 1.9.3 & 2.0.0-beta.8
Angular: 4.2.4
Bootstrap: 4.0.0-beta2(latest)

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

Angular CLI

[brgaulin]

@IlyaSurmay was curious if you remember this one:

The error appears to be in this checkDropup() method: https://github.com/valor-software/ngx-bootstrap/blob/development/src/dropdown/bs-dropdown.directive.ts#L321

In chrome/firefox, const top on the second opening returns 0px, but in IE it returns auto. In chrome the dom when debugged has inline styles like left: 0px; right: auto; top: auto; transform: translateY(-101%); background-color: rgb(56, 123, 182); which have that top:auto. Not sure what that detection is for, but it appears wonky in IE11.

In IE11 getComputedStyle returns 'auto' since it is on the inline style.

I'll send a PR for discussion in a second, but it appears that if I remove that top auto check, it functions as a drop up perfectly fine. Dropdowns too