QTextCursor::mergeCharFormat unable to change font family when QTextCharFormat with default QFont passed as an arg

If a QTextDocument is given html content, attempting to change the font family by calling QTextCursor::mergeCharFormat fails consistently in changing the family if the QTextCharFormat passed as an argument has a default QFont and the font family is set by calling QTextCharFormat::setFontFamily.

The issue is fixed by calling QTextCharFormat::setFont before passing it to mergeCharFormat.

Steps to reproduce:

1. Run the attached project
2. The sample project will attempt to change the font to jokerman
3. Monitor the output html => the span containing "Hello world" will have a wrong font family set
4. (optional) Pass "-workaround" as an arg to apply workaround for comparison.

Note: This problem does not appear in Qt 5.12.