bugfix for XML exporter

Bug: MARC XML exporter does not set xml declaration or collection element

    $ catmandu convert MARC --type RAW to MARC --type XML < ./t/camel.usmarc
    <marc:record><marc:leader>00755cam  22002414a 4500</marc:leader>

Added bugfix to Catmandu::Exporter::XML and 13-marcxml.t.