Attachments flag is dropped when applying delta to incoming rev

When an incoming rev is a delta, `IncomingRev::parseAndInsert` can either decide to let the `Inserter` apply the delta later, or to apply the delta itself now. However, there is a crucial piece of code missing when the delta is applied immediately which means if; there are no blob changes in the delta, and the doc has existing blobs - the attachments flag is not applied to the new incoming rev.

The fix is pretty simple: Add a check in `IncomingRev::parseAndInsert` to see if the doc has any blobs, if it does add the `kRevHasAttachments` flag to the `IncomingRev->_rev.flags`