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>

Start with the referenced commit, especially the linked diff around line 87, and reproduce the append sequence shown in the issue. Check the EmbeddedDocument behavior before and after adding it to the parent list; done means _instance is populated immediately after append, with a regression test covering that case.