HTTP::OAI::MetadataFormat - Encapsulates OAI metadataFormat XML
    data
  - $mdf = new HTTP::OAI::MetadataFormat
- This constructor method returns a new HTTP::OAI::MetadataFormat
    object.
- $mdp = $mdf->metadataPrefix([$mdp])
- $schema = $mdf->schema([$schema])
- $ns = $mdf->metadataNamespace([$ns])
- These methods respectively return and optionally set the metadataPrefix,
      schema and, metadataNamespace, for the metadataFormat record.
    metadataNamespace is optional in OAI 1.x and therefore may be
        undef when harvesting pre OAI 2 repositories.