Delete doesn't work with assigned id name strategy

Delete seems not to work in this test case. Works fine if you comment out the static mapping for id generator...
```groovy
class AssignedIdDeleteSpec extends Specification {

@Shared
@AutoCleanup
Neo4jDatastore datastore = new Neo4jDatastore(ThingToDelete)

@Rollback
void "delete"() {
given:
def thing = new ThingToDelete(name: "thing1")
thing = thing.save(flush: true)
thing = thing.delete()

expect:
def found = ThingToDelete.findByName("thing1")
found == null
}
}

@Entity
class ThingToDelete {
String name
static mapping = node {
id {
generator "assigned"
name "name"
}
}
}
```

Start by reproducing the issue with the AssignedIdDeleteSpec example and the Neo4jDatastore configured for ThingToDelete. Trace delete handling when the id generator is assigned and the mapping names the id property; done means ThingToDelete.findByName("thing1") returns null after deletion.

Written by the indexing model from the issue text.