Issue title: /2/groups.json endpoint has stopped returning topics
 Issue body: Authentication type: API key in request URL
Endpoint: `/2/groups.json`

Per the API docs for the `/2/groups` endpoint, `topics` should be a collection returned in the response body for a given group: http://www.meetup.com/meetup_api/docs/2/groups/

However, responses from this endpoint appear to be missing the `topics` collection altogether, for all results.

A sample request: `curl -X GET "https://api.meetup.com/2/groups.json/?zip=90017&radius=50&key=xxxx&page=1"`

For reference, the endpoint was working properly as of Feb 27. Thanks for your help!
 Comments: 
 Comment 0: It looks as though this has happened at least once before: https://groups.google.com/forum/#!searchin/meetup-api/topics/meetup-api/fJZnGKV2-GI/QENPwr362GkJ
 Comment 1: Hi @devmage,

This looks like a recent regression. We recently started translating topic names and I think the bug was introduced then. I'll open a ticket.

In the meantime, you can work around the issue by appending `fields=topics` to your requests. That should append the topics to group responses again.

Sorry for the inconvenience.
 Comment 2: Thanks for the tip about `fields`! It appears to restore the missing `topics` collection. Easy workaround.

Good luck in the translation effort!
 Comment 3: ok. this should now be fixed.
 Comment 4: Yes, it looks to be so. Thanks again!
 Comment 5: yep. sry about that
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