Map Distribution formats to something that CKAN understands

[amercader]

Harvested DCAT metadata will have defined as media types (eg application/xml). In order for previews to work in CKAN, resource formats need to be defined as xml.

Eventually CKAN core should support media types (see ckan/ckan#1350 and ckan/ckan#1336), but in the meantime the extension could do the mapping automatically, perhaps using this:

https://github.com/ckan/ckan/blob/master/ckan/config/resource_formats.json

[amercader]

The RDF harvester handles this, setting up resource['format'] and resource['mimetype'] accordingly