#295 let entity save accept null value for attribute

https://github.com/GoogleCloudPlatform/gcloud-python/issues/295 

Has unit test
This is a bit much.

Shouldn't we assume that `value_pb` only has one field set or none at all?

Both `value_pb.ListFields()` and `value_pb._fields` could be used. A quick check of `value_pb._fields` in all of our tests shows it is never set before this method is called.

Also, since null may be confused as a default, we may be better off using a sentinel value, i.e.
```python
NULL_PROPERTY = object()
```

and documenting `helpers.NULL_PROPERTY` as the value to use.