Fix image ids in API output

[chrismytton]

Make sure the images ids in the API output are correct. When there is no schema defined the API doesn't know what to do with the _id field, so it serializes it to an object instead of a string.

Eventually I'd like all of the schemas in popit-api to be explicit like this (see https://github.com/mysociety/popit/issues/264#issuecomment-85448579 for more details on whats involved with that).

Fixes https://github.com/mysociety/popit/issues/470

[chrismytton]

This might also fix https://github.com/mysociety/popit-api/issues/83

[struan]

Looks fine.