grouped p-dropdown value setting is not working using [(ngModel)]

[ASK83]

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Current behavior
When using grouped p-dropdown the value cannot be set using ngModel.

Expected behavior
Whenever there is a predefined value linking the p-dropdown to a value the value should be set like the regular p-dropdown

Minimal reproduction of the problem with instructions
Here is the link I am showing the issue with grouped p-dropdown vs simple p-dropdown
http://plnkr.co/edit/RNHTvMjGuPXEgmpqC7hE?p=preview

What is the motivation / use case for changing the behavior?
It is a common use case to have default values for the dropdown that needs to be set.

Please tell us about your environment:

• Angular version: 2.0.X
  @angular 5.2.5

• PrimeNG version: 2.0.X
  5.2.0

• 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 ]
  All of them

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

• Node (for AoT issues): node --version =
  8+

[cagataycivici]

Fixed via;

#5080