Bug fix: In RDF update, delete all old triples

[demery]

The change to not send empty values to RDF when objects are updated has meant that old triples are not getting deleted.

In order to prevent triples without objects being added to the store, a change was made to only send to the interface those values that a present on objects. Thus:

  def to_rdf
    map = {
      model_class: "names",
      id: id,
      fields: {}
    }

    map[:fields][:name]       = "'''#{name.to_s.gsub("'", "")}'''"       if name.present?
    map[:fields][:viaf_id]    = "'''#{viaf_id}'''"                       if viaf_id.present?
    map[:fields][:subtype]    = "'''#{subtype}'''"                       if subtype.present?
    map[:fields][:startdate]  = "'''#{startdate}'''"                     if startdate.present?
    map[:fields][:enddate]    = "'''#{enddate}'''"                       if enddate.present?
    map[:fields][:other_info] = "'''#{other_info.to_s.gsub("'", "")}'''" if other_info.present?
    map[:fields][:deleted]    = "'#{deleted}'^^xsd:boolean"              unless deleted.nil?

    map
  end

However, the RDF update process deletes each triple before inserting the updated one. interface.rb deletes only those triples given in the message hash:

      message['fields'].each do |field, new_value|
        query += %Q(
          DELETE { ?subject #{predicate} ?object }
          INSERT { ?subject #{predicate} #{new_value} }
          WHERE {
            BIND (<https://sdbm.library.upenn.edu/#{message['model_class']}/#{message['id']}> as ?subject) .
            OPTIONAL { ?subject #{predicate} ?object }
          };
        )
      # etc.
      end

Since empty values are not sent in the message hash, the delete clause isn't created. To fix this, the empty fields should be sent in the message hash, and -- in the update code in interface.rb -- added to the DELETE clause, but no INSERT clause should be created.