feat(entity): Create child when new, upon adding

src/entity.js

@@ -140,24 +140,40 @@ export class Entity {
  /**
   * Add an entity to a collection (persist).
   *
   * When given entity has data, create the entity and set up the relation.
   *
   * @param {Entity|number} entity     Entity or id
   * @param {string}        [property] The name of the property
   *
   * @return {Promise}
   */
  addCollectionAssociation(entity, property) {
    property    = property || getPropertyForAssociation(this, entity);
    let idToAdd = entity;
    let body    = undefined;
    let url     = [this.getResource(), this.id, property];

    if (entity instanceof Entity) {
      if (!entity.id) {
        return Promise.resolve(null);
      }
    if (this.isNew()) {
      throw new Error('Cannot add association to entity that does not have an id.');
    }

    if (!(entity instanceof Entity)) {
      url.push(entity);

      idToAdd = entity.id;
      return this.getTransport().create(url.join('/'));
    }

    return this.getTransport().create([this.getResource(), this.id, property, idToAdd].join('/'));
    if (entity.isNew()) {
      // Entity is new! Don't supply an ID, and just pass in the child.
      body = entity.asObject();
    } else {
      // Entity isn't new, just add id to url.
      url.push(entity.id);
    }

    return this.getTransport().create(url.join('/'), body)
      .then(created => {
        return entity.setData(created).markClean();
      });
  }

  /**