magento/magento2#17635: [Forwardport] addExpressionFieldToSelect has to be called after all addFieldToSelect.

[p-bystritsky]

Original pull request: #17915

Title

Fix Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection method addExpressionFieldToSelect result gets overwritten by addFieldToSelect.

Description

Fix Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection method addExpressionFieldToSelect result gets overwriten if called before addFieldToSelect. It happened in _initSelectFields method, if some columns were set before that method call they were overwirtten by it.
I changed the behaviour of addExpressionFieldToSelect method to not modify columns and instead insert expression into _fieldsToSelect private variable (same as addFieldToSelect does).

Fixed Issues (if relevant)

1. addExpressionFieldToSelect has to be called after all addFieldToSelect #17635: addExpressionFieldToSelect has to be called after all addFieldToSelect

Manual testing scenarios

-create a collection
$bookingCollection = $this->_bookingCollectionFactory->create();
$bookingCollection->addExpressionFieldToSelect('stime_part', 'time(start_time)', []);
$bookingCollection->addFieldToSelect('finish_time', 'finish_date');

Expected result after the fix.
Field 'stime_part' populated with calculated data.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @p-bystritsky. Thank you for your contribution
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[VladimirZaets]

Hi @p-bystritsky. The PR with the port for this issue was already created.
Thanks