Add tests for Bio::PubMed#search, query, pmfetch

 * Add tests for Bio::PubMed#search, query, pmfetch, although
   they will be deprecated in the future.
 * This commit and commit bfe4292c51bba5c4032027c36c35e98f28a9605a
   are inspired by https://github.com/bioruby/bioruby/pull/76
   (though the pull request have not been merged), and the commits
   fix the issue https://github.com/bioruby/bioruby/issues/75.
   Thanks to Paul Leader who reported the issue and the pull request.