Fix segfault when doing marking and doc is not a document node

This fixes the problem where the doc node can actually also be a
XML_DOCUMENT_FRAG_NODE. In that case it's structured like a node which
means it stores a VALUE inside _private.

This checks the type of the code and uses the correct marking for the
specific cases.

Fixes #939
Merge pull request #1006 from dbussink/fix_nokogiri_segfault_on_non_document_nodes

Fix segfault when doing marking and doc is not a document node