DateTimeProperty auto_now* is not set for StructuredProperty and LocalStructuredProperty #216

1. OS X
2. Python 3.7.3
3. $ pip show google-cloud-ndb
   Name: google-cloud-ndb
   Version: 0.1.0
   Summary: NDB library for Google Cloud Datastore
   Home-page: https://github.com/googleapis/python-ndb
   Author: Google LLC
   Author-email: googleapis-packages@google.com
   License: Apache 2.0
   Location: /Users/tcross/development/virtual-environments/csgapi37/lib/python3.7/site-packages
   Requires: google-cloud-datastore, redis

Steps to reproduce

1. Create a Model with a DateTimeProperty with auto_now_add=True, use that model in another model as a StructuredProperty or LocalStructuredProperty. Create an instance of the model that has the StructuredProperty and then save it. The StructuredProperties entities DateTimeProperty will have a value of None instead of a date time.

Code example

Foo(ndb.Model):
    dtp = ndb.DateTimeProperty(auto_now_add=True)

Bar(ndb.Model):
    foo = ndb.StructuredProperty(Foo)

bar = Bar()
bar.foo = Foo()
bar.put()

Stack trace

N/A