db_field isn't used if field is not present in document, leading to validation errors

This bug has arisen from changing the db_field of a value due to the new schema of that value having validation errors, in an attempt to migrate the data.

It seems that if the document does not have the key db_field it will fall back to the field name still, which is somewhat unexpected behaviour and can cause unexpected validation errors.

As this behaviour is not described in the documentation at all, it should at least be put in as warnings if not resolved.

The below code reproduces the bug described.

from mongoengine import *

class TestDocument(Document):
    attributeName = StringField(db_field="fieldName")

if __name__ == '__main__':

    db = connect('Issue#1102')
    db.drop_database('Issue#1102')

    # Create Test Document
    d = TestDocument()
    d.save()

    # get and validate object
    d2 = TestDocument.objects().first()
    # VALID
    d2.validate()

    # Add in bad field
    TestDocument.objects.update(__raw__={
        "$set": {
            "attributeName": 1
        }
    })

    # get and validate object
    d3 = TestDocument.objects().first()

    # FAILS
    try:
        d3.validate()
    except ValidationError as e:
        print("Failed d3.validate(). %s" % e)

    # Add in good field
    TestDocument.objects.update(__raw__={
        "$set": {
            "fieldName": "string"
        }
    })

    # get and validate object
    d4 = TestDocument.objects().first()
    # VALID
    d4.validate()

Start by running the provided Python reproducer against MongoEngine and compare validation when the document contains attributeName versus its db_field, fieldName. Trace the loading and validation path for db_field handling; done means the reproduced case no longer produces the unexpected validation error, or the fallback behavior is explicitly warned about and documented.