AMQP-230: DirectExchange: allow omit `key` attr

JIRA: https://jira.springsource.org/browse/AMQP-230

Previously, `key` attribute for `direct-exchange` `binding` could be omitted,
but that meant that the real binding was done with empty `routingKey`.
With this change the empty `routingKey` is allowed only, if `key=""`.
Omitting `key` means that binding will be done by the name of provided `Queue` or `Exchange`