Back in b92660e (#1834 fixing #1063) I omitted support in JRuby for
the "new_parent_document" argument to `Node#dup` because there was no
performance reason to implement it. So the test was skipped.

However, in 1e7d38a and other commits in #3117 (fixing #316), I
introduced a call to `initialize_copy_with_args` that passes the new
parent document as an argument on both CRuby and JRuby
implementations. Because the test was skipped, I didn't catch that
this broke on JRuby.

In particular this was a problem for Loofah which relies on
decorators, and even more particularly this broke the
`Loofah::TextBehavior` formatting concern for
`Loofah::*::DocumentFragment` objects.

Maybe we should be running downstream tests with JRuby, too? But that
feels like a big investment right now so I'll avoid scarring on the
first cut, and wait to see if it happens again.

(cherry picked from commit dda0be2)