Customer group extension attributes not carried over on save

[JosephMaxwell]

Description

When saving a customer group, extension attributes should be copied to the new data model that is returned. Unfortunately, this action was not completed and results in unexpected behavior.

Fixed Issues (if relevant)

Manual testing scenarios

1. Ensure that saving a customer group does not throw any errors.
2. Ensure that utilizing extension attributes for a customer group now carries over through the save process.

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 @JosephMaxwell. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
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[JosephMaxwell]

@phoenix128: do you know why the Travis tests haven't built yet?

[phoenix128]

Hello @JosephMaxwell ,
we apologize for this, we are experiencing some slowdowns due to the high amount of PR.

Do not worry, I will take care of you PR :)

[phoenix128]

Thank you @JosephMaxwell for your contribution. Your help is appreciated.

[magento-engcom-team]

Hi @phoenix128, thank you for the review.
ENGCOM-2089 has been created to process this Pull Request

[magento-engcom-team]

@JosephMaxwell thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @JosephMaxwell. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
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.