Reinstate dimensions in the _id field

Because they are a unique part of the record. Removing it in previous
commit 7cf43e52f17c709d2c4035537b2f2bbb16de5143 caused a regression:
https://www.pivotaltracker.com/story/show/63703882

Note that I'm planning on making the ``_id`` field human-readable, instead of base64 encoded, but in the interests of getting this bug fixed ASAP I'll do that in a separate pull request.