Use rdf_value and replace mailto prefix

[maudetes]

Supports OpenDataSoft contactPoint description, ex:

<dcat:contactPoint>
    <vcard:Organization>
        <rdf:type rdf:resource="http://www.w3.org/2006/vcard/ns#Kind" />
        <vcard:hasEmail rdf:resource="mailto:contact@agenceore.fr" />
    </vcard:Organization>
</dcat:contactPoint>

We make sure to use the string mailto:contact@agenceore.fr as value using rdf_value and then we strip mailto: prefix