Server returns search continuation only if 'ref' or '+' attributes are requested

If the search scope includes referral entries, then the server only converts them to search continuations if the search requested '+' or 'ref' attributes, otherwise the referral entry is returned.

In 2.0.0-M14 this was different, even if only objectClass was requested by the client the referral object was converted to a search continuation.

Unfortunately our tests don't cover this case.

• SearchWithReferralsTest always requests * and +. ReferralSearchIT
• ReferralSearchIT.testSearchContinuations() checks ReferralException but that catch block is never reached.