ISO-19139 URN `fileIdentifier` modified when converted to DCAT

[streino]

Describe the bug

Conversion from ISO-19139 to DCAT modifies URN fileIdentifiers. The "urn:..." fileIdentifier becomes "urn/..." in DCAT.

There is an inconsistency between the (ISO-19115-3.2018) codeSpace extraction (stores "urn" without separator) and the way DCAT puts it back together (checks "urn:" with separator).

Example record: https://www.geo2france.fr/geonetwork/srv/fre/catalog.search#/metadata/urn:isogeo:metadata:uuid:e9dc2a55-5783-481e-8582-aca1461bda5b

Original ISO-19139:

<gmd:fileIdentifier>
  <gco:CharacterString>urn:isogeo:metadata:uuid:e9dc2a55-5783-481e-8582-aca1461bda5b</gco:CharacterString>
</gmd:fileIdentifier>

DCAT output:

<foaf:isPrimaryTopicOf>
  <rdf:Description rdf:about="urn/isogeo:metadata:uuid:e9dc2a55-5783-481e-8582-aca1461bda5b">

Intermediate ISO-19115-3 representation:

<mdb:metadataIdentifier>
  <mcc:MD_Identifier>
     <mcc:code>
        <gco:CharacterString>isogeo:metadata:uuid:e9dc2a55-5783-481e-8582-aca1461bda5b</gco:CharacterString>
     </mcc:code>
     <mcc:codeSpace>
        <gco:CharacterString>urn</gco:CharacterString>
     </mcc:codeSpace>
  </mcc:MD_Identifier>
</mdb:metadataIdentifier>

Expected behavior

URN identifier should be preserved as-is.