Fragment type conditions don't work for union types

[wravery]

Take the following example from the schema.today.graphql sample:

{
	appointments {
		edges {
			node {
				...AppointmentUnionFragment
			}
		}
	}
}

fragment AppointmentUnionFragment on UnionType {
	...on Appointment {
		appointmentId: id
		subject
		when
		isNow
	}
}

The AppointmentUnionFragment fragment never matches on the Appointment type.