Added form fieldset before html data to \Magento\Framework\Data\Form\Element\Fieldset in getElementHtml() method

[vasilii-b]

Description

In \Magento\Framework\Data\Form\Element\Fieldset class, there is not called the getBeforeElementHtml() method in getElementHtml(). This stops developers from adding needed information before the fieldset in a easy way.

Fixed Issues (if relevant)

1. Class \Magento\Framework\Data\Form\Element\Fieldset breaks specification of the parent class \Magento\Framework\Data\Form\Element\AbstractElement by not calling the method getBeforeElementHtml (getAfterElementHtml is called) #2618

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 @vasilii-b. Thank you for your contribution
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3268 has been created to process this Pull Request

[magento-engcom-team]

Hi @vasilii-b. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.