Fill _instance when adding an embedded document to a list

EmbeddedDocument._instance is not filled when the embedded document as added to a document's list of embedded docs:
https://github.com/MongoEngine/mongoengine/commit/7d90aa76ff7116269dea42f2c6629ea6b868b0de#diff-da780e321f79f4602bab26240c7b0d7dR87

ipdb> l
    136         benefit = PurchaseBenefit()
    137         coupon.benefits.append(benefit)
    138         # HACK: force MongoEngine to fill _instance
    139         import ipdb; ipdb.set_trace()
--> 140         coupon.benefits[0]
    141     form = PurchaseBenefitForm(request.form if request.method == 'POST' else None, obj=benefit)
    142     errors = []
    143     if request.method == 'POST' and form.validate():

ipdb> isinstance(benefit, mongoengine.EmbeddedDocument)
True
ipdb> benefit._instance
ipdb> n
> /Users/Vic/projects/brain2/brain/crm/coupons.py(141)edit_coupon_benefit()
    140         coupon.benefits[0]
--> 141     form = PurchaseBenefitForm(request.form if request.method == 'POST' else None, obj=benefit)
    142     errors = []

ipdb> benefit._instance
Coupon(id='5b915f6a2ac6b371ed8f21df', ...)
ipdb>

Empieza por el commit referenciado, especialmente por el diff enlazado alrededor de la línea 87, y reproduce la secuencia de append mostrada en el issue. Comprueba el comportamiento de EmbeddedDocument antes y después de añadirlo a la lista principal; se considera terminado cuando _instance está rellenado inmediatamente después de append, con una prueba de regresión que cubra ese caso.