Add failing test for mutating DOMDocument
Add failing test for mutating DOMDocument

A DOMDocument instance being passed to the parser should not have
changed after parsing. This could potentially trip-up further use of
the same DOMDocument instance.

See https://github.com/indieweb/php-mf2/issues/174.

gRegorLove/php-mf2,gRegorLove/php-mf2