Nested objects have to be set as "embedded" even when they are all in src/main/groovy.

Nested objects have to be set as "embedded" to be persisted even when they are all in src/main/groovy. I believe non entity classes should always be saved as embedded objects. Above all, non entity objects should never have to have `static embedded` declarations.

The following test only pass when the commented lines get uncommented. I believe it would be more appropriate if it passed even without those commented lines.

```package org.grails.datastore.gorm.mongo

import grails.gorm.tests.GormDatastoreSpec
import grails.persistence.Entity

class EmbeddedWithinEmbeddedSimpleObjectSpec extends GormDatastoreSpec {

void "Test embedded nested non-domain object"() {
when:"An entity with a simple nested non-domain embedded object is persisted"
def constellation = new Constellation(displayName: "foo", solarSystem: new SolarSystem(planet: new Planet(name: "earth")))
constellation.save(flush:true)
session.clear()
constellation = Constellation.get(constellation.id)
then:"The embedded association is persisted correctly"
constellation.solarSystem?.planet?.name == 'earth'
}

@Override
List getDomainClasses() {
[Constellation]
}
}

@Entity
class Constellation {

String id
String displayName

SolarSystem solarSystem

static embedded = [ 'solarSystem' ]
}

//@Entity
class SolarSystem {
Planet planet

// static embedded = [ 'planet' ]
}

//@Entity
class Planet {
String name
}
```