Fix support for element_children in document fragments.

libxml2 doesn't support finding children in document fragments, but I don't think there's a good reason for this. Bug and proposed patch for libxml2 here:

https://bugzilla.gnome.org/show_bug.cgi?id=733900

This addresses Nokogiri #1138.

Unfortunately, I haven't been able to get comment on the patch from upstream libxml2 on either the bug or [a message](https://mail.gnome.org/archives/xml/2014-July/msg00003.html) about it.