Fixed OpenBeelden OAI extractor

OpenBeelden uses the resumption token to encode the metadataPrefix, rather than a separate HTTP GET parameter. Enclosed is a fix that at least enables us to pull in their data again, although it is not entirely according to the [OAI-spec](http://www.openarchives.org/OAI/2.0/openarchivesprotocol.htm#MetadataNamespaces).