[Backport] [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)

[hitesh-wagento]

Hi @VladimirZaets

Any update on this PR ?

Thanks

[hitesh-wagento]

Hi @magento-engcom-team

Any update in this PR ?

Thanks

[sidolov]

Hi @hitesh-wagento , our QA reported that provided changes cause an error during the import.