Ensure we prefix xpath with // for searching entire document

Tests prior to this commit passed because libxml2 was not so
strict. Since upgrading to 2.9.0 this issue has been revealed.
The former test should have been written with this in mind.

Steps to reproduce (on OSX with Homebrew installed):

  gem uninstall nokogiri
  brew install libxml2
  LDFLAGS="-L/usr/local/opt/libxml2/lib" \
    CPPFLAGS="-I/usr/local/opt/libxml2/include" \
    gem install nokogiri

closes #308