Fix/add expresion

[magently]

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]

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[sidolov]

Hi @magently , please, update the PR with the issue description and steps to reproduce, otherwise, we can't proceed with your PR.
Thank you!

[magently]

Sorry about the description, i was certain i filled it before. Updated

[magento-engcom-team]

Hi @slavvka, thank you for the review.
ENGCOM-3086 has been created to process this Pull Request

[magento-engcom-team]

Hi @magently. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.