spurious data fields are added to an uploaded image

[mhl]

For example, I uploaded an image for this person:

• https://mark-test.popit.mysociety.org/api/v0.1/persons/55115704658b705d703c8719

... with the command:

curl -v -X POST \
    --header "APIKey: abcdef0123456789" \
    -F notes='Official Portrait' \
    -F source='Found it somewhere' \
    -F image='@Desktop/svens.png' \
    https://mark-test.popit.mysociety.org/api/v0.1/persons/55115704658b705d703c8719/image

... but the corresponding object in the images array contains these spurious fields:

        "start_date": null,
        "end_date": null,
        "birth_date": null,
        "death_date": null,
        "founding_date": null,
        "dissolution_date": null

[mhl]

Actually, I think this is a duplicate of https://github.com/mysociety/popit/issues/714