Update Html.php to enable choice of leading method.

[macrini]

The current loadXML() rejects valid self-closing elements from HTML. The problem can be fixed in a backwards compatible way by adding a new boolean argument to choose which loading method to use.

Description

Self-closing tags like <br> and <img> are valid in HTML but not in XML. Loading the DOM via loadHTML() solves the current problem with the static function addHtml(). Since the loading method is a choice that depends on the caller, a new boolean argument is added to control which method will be used, defaulting to the current behaviour which is to use loadXML().

Fixes # (issue)

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes