For possible future usage allow multiple checksums

Instead of returning

<oc:checksum>TYPE:CHECKSUM</oc:checksum>

Return

<oc:checksums>
  <oc:checksum>TYPE:CHECKSUM</oc:checksum>
</oc:checksums>

This will allow us to expand in the future to multiple checksums.
Without doing just string concatenation.

And even for a single checksum it is now correct.