some PopIt-hosted images 404 when accessed via the image proxy

[mhl]

This was pointed out by @jarofgreen - for this person:

• http://yournextmp.popit.mysociety.org/api/v0.1/persons/1476

... the image works fine:

• https://yournextmp.popit.mysociety.org/persons/1476/image/5517f1db249b917507bee502

... but the proxy_image returns a 404:

• https://yournextmp.popit.mysociety.org/image-proxy/http%3A%2F%2Fyournextmp.popit.mysociety.org%2Fpersons%2F1476%2Fimage%2F5517f1db249b917507bee502

[mhl]

I should have spotted this, but as @chrismytton pointed out, this is the common source of confusion that the proxy_image needs to have a size specification after it, e.g. /80/0 to specify that it should have width 80 and whatever height then follows from preserving the aspect ratio:

• https://yournextmp.popit.mysociety.org/image-proxy/http%3A%2F%2Fyournextmp.popit.mysociety.org%2Fpersons%2F1476%2Fimage%2F5517f1db249b917507bee502/80/0