Bug fix: Bio::PubMed#efetch should return an array of string objects.

 * Bug fix: Bio::PubMed#efetch should return an array of string
   objects. Thanks to Masahide Kikkawa and Fredrik Johansson
   who report the bug (in "[BioRuby] Bio::PubMed.efetch, bug?"
   and "[BioRuby] PubMed.efetch error").

lib/bio/io/pubmed.rb

@@ -117,7 +117,11 @@ def esearch(str, hash = {})
   def efetch(ids, hash = {})
     opts = { "db" => "pubmed", "rettype"  => "medline" }
     opts.update(hash)
-    super(ids, opts)
+    result = super(ids, opts)
+    if !opts["retmode"] or opts["retmode"] == "text"
+      result = result.split(/\n\n+/)
+    end
+    result
   end
 
   # Search the PubMed database by given keywords using entrez query and returns