the _id for images is odd

[mhl]

If you look at the images for a person in PopIt, e.g. https://yournextmp.popit.mysociety.org/api/v0.1/persons/47 there's a surprising _id element included (in the id in there isn't hex, and it's non-ASCII):

    "_id": {
      "_bsontype": "ObjectID",
      "id": "Tèà±Pâ8p,\u0006\r"
    }

... and presumably there's no need to expose the _bsontype

[chrismytton]

Dupe of https://github.com/mysociety/popit/issues/470 (which I reopened the other day)