Add DNS keys to domain API response

It is fairly similar to what we do for WHOIS records, but one query is
slightly rewritten with removal of try-catch block

lib/serializers/registrant_api/domain.rb

@@ -42,11 +42,23 @@ module Serializers
           locked_by_registrant_at: domain.locked_by_registrant_at,
           status_notes: domain.status_notes,
           nameservers: nameservers,
+          dnssec_keys: dnssec_keys,
+          dnssec_changed_at: dnssec_updated_at
         }
       end
 
       private
 
+      def dnssec_keys
+        domain.dnskeys.map do |key|
+          "#{key.flags} #{key.protocol} #{key.alg} #{key.public_key}"
+        end
+      end
+
+      def dnssec_updated_at
+        domain.dnskeys.order(updated_at: :desc).select(:updated_at).first
+      end
+
       def contacts(type)
         contact_pool = begin
                          if type == :tech