markdown-update-header-faces needed explicit call on startup

[dickmao]

Description

User saves customization of markdown-header-scaling and restarts emacs only to find headers do not resize because the :set attribute of defcustom markdown-header-scaling is not explicitly called.

Rationalize the defcustom stuff and test.

Related Issue

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Improvement (non-breaking change which improves an existing feature)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have read the CONTRIBUTING.md document.
• [ ] I have updated the documentation in the README.md file if necessary.
• [ ] I have added an entry to CHANGES.md.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed (using make test).