fix multiple harvesting of first record set

Bug: first result set is imported two times
 
    $ curl "http://services.d-nb.de/sru/zdb?version=1.1&operation=searchRetrieve&query=tit%3Dsoil+and+biology&recordSchema=MARC21-xml" | grep -o "<numberOfRecords>.*</numberOfRecords>"
    <numberOfRecords>23</numberOfRecords>
    $ catmandu convert -v --base http://services.d-nb.de/sru/zdb --query "soil and biology" --recordSchema MARC21-xml
    converted 33 objects
    done

Fixed bug, added test and moved XML files to /t/files.