Provide a way to explicitly set fields to null about protorpc HOT 2 CLOSED

Digging a bit further, I'm not certain if this issue should be filed here or in the google/apitools repository.

I've found that some fields actually encode this as expected -- our Object messages contain a separate metadata field -- this is a repeated field containing instances of a separate message type, MetadataValue, which is decorated with an apitools function called MapUnrecognizedFields. It seems that if you a supply a value of None for one of these unrecognized fields, it will be JSON-encoded as null, rather than omitting the key/value pair altogether.

As an example, when encoding an Object message containing one of these (with a key of "foo" and a value of None) to JSON, the value of null will be present, rather than omitting the attribute:

(Pdb) obj.metadata = apitools_messages.Object.MetadataValue()
(Pdb) obj.metadata.additionalProperties = []
(Pdb) obj
<Object
 acl: []
 metadata: <MetadataValue
 additionalProperties: []>>

(Pdb) obj.metadata.additionalProperties.append(apitools_messages.Object.MetadataValue.AdditionalProperty(key='foo', value=None))
(Pdb) obj
<Object
 acl: []
 metadata: <MetadataValue
 additionalProperties: [<AdditionalProperty
 key: 'foo'>]>>

(Pdb) encoding.MessageToJson(obj)
'{"metadata": {"foo": null}}'

Although this encoding module is defined within apitools here, and looks to be its own extension of protorpc's ProtoJson class, so probably outside the scope of this repository.

from protorpc.

Upon further investigation, it looks like apitools has an IncludeFields method that takes care of this:
https://github.com/google/apitools/blob/2bfa257a65e335ef67afc9f135c7796f47f85535/apitools/base/py/base_api.py#L356

from protorpc.