\Drupal\Core\Datetime\DateHelper::weekDaysOrdered() does not preserve keys when reordering

Problem/Motivation

When a custom "First day of week" is chosen, DateHelper::weekDaysOrdered() rekeys the result, so the 0th day corresponds to the first day.
This breaks expectations of projects like Calendar and Station module, who still internally consider Sunday the 0th day, and work with the "first day" setting.

Proposed resolution

Do not rekey when reordering.

Remaining tasks

N/A

User interface changes

N/A

API changes

Barely.

Data model changes

N/A

Important for RC: contributed project blocker. This code was moved into core for D8, and ideally should be correct.