[Propel1] Fixed preferred choices in the ModelChoiceList

Related change was made in #6454 but for some reason it was not merged from 2.1 branch.

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: some Locale tests fail, fixed in #6568 
Fixes the following tickets:
Todo: -
License of the code: MIT
Documentation PR: -