Fix tests

A new API version has been released
and the data schema did change.

tests/data/plain.xml

@@ -11,6 +11,7 @@
       </authors>
       <editors/>
     </contributors>
+    <database>Zbl</database>
     <document_type>journal article</document_type>
     <editorial_contributions>
       <contribution_type>review</contribution_type>