beta:indicativeOffer should have optional @id

[matdavies]

Given a FacilityUse with the following offers property:

"offers": 
[
  {
    "@type": "beta:IndicativeOffer",
    "identifier": "ven_000229-rbt_000010-rc_000051",
    "name": "Five A Side",
    "price": 67.89,
    "priceCurrency": "GBP"
  }
]

The online validator and validator library flags the following validation error:

- Error: FAILURE: $.facilityUse.offers[0].id: Required property `@id` is missing from `Offer`.

Seems to be applying the rules for Offer instead of those for beta:indicativeOffer. beta:indicativeOffers without @id should be allowed.