[Forwardport] [Fix] forgot to add lowercase conversion on grouped product assignation

[hitesh-wagento]

Original Pull Request

#15312
Forgot to add lowercase conversion on grouped product assignation after changes done in MAGETWO-67240

• See changes done for MAGETWO-67240 here 02596b7#diff-b876c962d83b3bee42acba83af842309

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 @hitesh-wagento. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

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[hitesh-wagento]

Hi @sidolov

Any update on this PR ?

Thanks

[sidolov]

Hi @hitesh-wagento , your PR awaiting for delivery.

[magento-engcom-team]

Hi @hitesh-wagento. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.