Choice()'s can take a string for their vocabulary, but lazr.restful can't handle this

[Barry Warsaw]

I have the following field defined in my IMessageApproval interface:

    status = exported(Choice(
        title=_('Status'),
        description=_('The status of the held message.'),
        vocabulary='PostedMessageStatus',
        required=True))

Zope has no problem with this, but lazr.restful cannot find the
PostedMessageStatus enum:

    ...
      File "/home/barry/projects/launchpad/pristine/lib/zope/component/_api.py", line 207, in getUtility
    ComponentLookupError: (<InterfaceClass zope.schema.interfaces.IVocabularyFactory>, 'PostedMessageStatus')

However, if I change the field to use the actual enum, it works fine:

    status = exported(Choice(
        title=_('Status'),
        description=_('The status of the held message.'),
        vocabulary=PostedMessageStatus,
        required=True))

Output from my test...

      disposal_date: None
      disposed_by_link: None
      mailing_list_link: u'http://api.launchpad.dev/beta/~team-one/mailing_list'
      message_id: u'<aardvark>'
      posted_by_link: u'http://api.launchpad.dev/beta/~name16'
      posted_date: u'2000-08-01T01:08:59+00:00'
      resource_type_link: u'http://api.launchpad.dev/beta/#message_approval'
      self_link: u'http://api.launchpad.dev/beta/~team-one/mailing_list/<aardvark>'
      status: u'New status'

In this particular case, it's fine to change the vocabulary argument from the
string to the enum, but this won't always be the case due to circular
references.

[Barry Warsaw (barry)]

Full traceback

    pprint_collection(messages)
Exception raised:
    Traceback (most recent call last):
      File "/home/barry/projects/launchpad/pristine/lib/zope/testing/doctest.py", line 1360, in __run
        save_stdout = sys.stdout
      File "<doctest xx-message-holds.txt[17]>", line 1, in ?
      File "/home/barry/projects/launchpad/385180-mmapi/lib/lazr/restful/testing/webservice.py", line 132, in pprint_collection
        collection = collection.jsonBody()
      File "/home/barry/projects/launchpad/385180-mmapi/lib/lazr/restful/testing/webservice.py", line 435, in jsonBody
        raise ValueError(self.getOutput())
    ValueError: HTTP/1.1 500 Internal Server Error
    Content-Length: 2990
    Content-Type: text/plain
    X-Lazr-Oopsid: OOPS-1320T17

    (<InterfaceClass zope.schema.interfaces.IVocabularyFactory>, 'PostedMessageStatus')

    Traceback (most recent call last):
      File "/home/barry/projects/launchpad/pristine/lib/zope/publisher/publish.py", line 133, in publish
      File "/home/barry/projects/launchpad/385180-mmapi/lib/lazr/restful/publisher.py", line 154, in callObject
        value = super(
      File "/home/barry/projects/launchpad/385180-mmapi/lib/canonical/launchpad/webapp/publication.py", line 373, in callObject
        return mapply(ob, request.getPositionalArguments(), request)
      File "/home/barry/projects/launchpad/pristine/lib/zope/publisher/publish.py", line 108, in mapply
      File "/home/barry/projects/launchpad/pristine/lib/zope/publisher/publish.py", line 114, in debug_call
      File "/home/barry/projects/launchpad/devel/lib/lazr/restful/_resource.py", line 820, in __call__
        result = self.do_GET()
      File "/home/barry/projects/launchpad/devel/lib/lazr/restful/_resource.py", line 1294, in do_GET
        result = self.handleCustomGET(operation_name)
      File "/home/barry/projects/launchpad/devel/lib/lazr/restful/_resource.py", line 628, in handleCustomGET
        return operation()
      File "/home/barry/projects/launchpad/pristine/lib/lazr/restful/_operation.py", line 59, in __call__
      File "/home/barry/projects/launchpad/pristine/lib/lazr/restful/declarations.py", line 833, in call
      File "/home/barry/projects/launchpad/pristine/lib/lazr/restful/_operation.py", line 94, in encodeResult
      File "/home/barry/projects/launchpad/eggs/simplejson-2.0.9-py2.4-linux-i686.egg/simplejson/__init__.py", line 236, in dumps
        separators=separators, encoding=encoding, default=default,
      File "/home/barry/projects/launchpad/eggs/simplejson-2.0.9-py2.4-linux-i686.egg/simplejson/encoder.py", line 200, in encode
        chunks = self.iterencode(o, _one_shot=True)
      File "/home/barry/projects/launchpad/eggs/simplejson-2.0.9-py2.4-linux-i686.egg/simplejson/encoder.py", line 260, in iterencode
        return _iterencode(o, 0)
      File "/home/barry/projects/launchpad/devel/lib/lazr/restful/_resource.py", line 131, in default
        return IJSONPublishable(obj).toDataForJSON()
      File "/home/barry/projects/launchpad/devel/lib/lazr/restful/_resource.py", line 1227, in toDataForJSON
        return self.toDataStructure(self.JSON_TYPE)
      File "/home/barry/projects/launchp...