ContentItem.tags

[nealrichardson]

From #160. If selecting an item from the results of .find(), tags may be present (and they will be by default now). So ContentItem.tags will return them if they're there. But if you just request a content item by guid, there's no option to include tags in the response. And we don't want to erroneously report them not to exist.

Proposal:

• If the GET v1/content?include=tags result does not contain tags for an item, we know it to be empty, so insert tags: [].
• In the tags @property on ContentItem, switch behavior on whether "tags" is in the data. If not present, so we should call the content item tags API, add them to the data in the object, and return them.

Following up in the Connect API to make things better going forward:

• GET v1/content?include=tags should always include a tags attribute for the items in the list, even if it is empty []
• GET v1/content/{guid} should accept include=tags (and any other things you can add to the catalog endpoint)

[tdstein]

As of #290, this can be simplified to adding a tags property, which sets the tags field if the value is missing or None.