v4.2.2

Appending an empty document fragment now returns the fragment correctly. This release also includes a regression test and a few improvements to the project's checks.

• 7d8b4590 Fix code coverage.
• 7374f679 Return empty document fragments correctly from appendChild.
• 0b2b4752 Update squizlabs/php_codesniffer from 3.13.5 to 3.13.6.