fix incorrect error class name

Author: george-carlin

lib/net/ldap/connection.rb

@@ -652,7 +652,7 @@ def add(args)
     pdu = queued_read(message_id)
 
     if !pdu || pdu.app_tag != Net::LDAP::PDU::AddResponse
-      raise Net::LDAP::ResponseMissingError, "response missing or invalid"
+      raise Net::LDAP::ResponseMissingOrInvalidError, "response missing or invalid"
     end
 
     pdu