Change _getHtml to append class rather than overwrite for children

[jonshipman]

Change _getHtml to append class rather than overwrite for children

Description

When creating a dependency injection for the Magento\Theme\Block\Html\Topmenu class, we are unable to change class names on children in a beforeGetHtml method because the protected method _getHtml declares setClass() on all children items. What this contribution changes is checking each child for an existing class and appends the $outermostClass if true.

Manual testing scenarios

1. Create a di.xml in a custom module that creates a plugin for the Magento\Theme\Block\Html\Topmenu class.
2. In a beforeGetHtml injection, add a loop over the (Magento\Theme\Block\Html\Topmenu) $subject->getMenu()->getChildren() and ->setClass() on each one.
3. With this contribution, each child gets the class specified. Without, it gets overwritten.

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-cicd2]

All committers have signed the CLA.

[dmanners]

@jonshipman thank you for the pull request. Could you sign the CLA and then we can start to progress this pull request.

[jonshipman]

I give my consent, the CLA button says I sign it but this PR doesn't seem to update. Attached is the image I see.

https://imgur.com/9mMXGXC

[dmanners]

@jonshipman it seems that the account the commit is assigned to (https://github.com/jonshipmansmc) is different from the account you are creating the pr and signing the CLA with (https://github.com/jonshipman). If you change the author or the commit and repush the branch it should pick up the changes and cla signing.

[jonshipman]

Signed.

[dmanners]

could you update this variable to be $currentClass