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
Merge branch 'kv_patch_for_frags' of https://github.com/kylev/nokogiri into kylev-kv_patch_for_frags
Merge branch 'kylev-kv_patch_for_frags'
Get the last patch formatted.
Merge "Fix regressions introduced by CVE-2014-0191 patch" from libxml2 v2.9.x.