Dropdown closing issue

[honcumilan]

I'm submitting a ... (check one with "x")

[X] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Plunkr Case (Bug Reports)
https://stackblitz.com/edit/github-2446y7?file=src/app/app.component.ts

Current behavior
The dropdown is still open after select one item by keyboard input. When i open dropdown and press S, the first element with prefix S is selected correctly, but dropdown after set disable to form control doesn't close correctly.

Expected behavior
Dropdown will be closed.

Minimal reproduction of the problem with instructions

1. click on dropdown
2. press keyboard [l,s,t key]
3. item will be selected but dropdown doesn't close correctly

• Angular version: 5.X
  11.0.4

• PrimeNG version: 5.X
  11.3.0

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
  probably all

• Language: [all | TypeScript X.X | ES6/7 | ES5]
  probably all

• Node (for AoT issues): node --version =
  probably doesn't matter