Return proper content type instead of image/*

[paskal]

Resolve #598.

Current master

➜ curl -o /dev/null -D - -s https://127.0.0.1:8080/api/v1/picture/github_f1f725d06af013456a9b98d923f9b2ce5c7ba91a/bq906dihajfi6fd90ci0 | grep '^Content-Type:'
Content-Type: image/*

After this PR

➜  remark42 git:(master) ✗ curl -o /dev/null -D - -s http://127.0.0.1:8080/api/v1/picture/dev_user/bq90856q9r8d70c29vpg | grep '^Content-Type:'
Content-Type: image/png

Also opening such an image in a browser now shows it instead of opening save prompt.

I've tested image save and load but didn't test proxy changes as it's not clear for me when it's involved. But I think it should work fine.

[paskal]

I can't figure out why this fails, working on it.

UPD: sometimes imageStore.Commit() is not called for some reason, I think I might fix it with imageStore.Close() call but for that I need fixes from #658 applied first.

[umputun]

LGTM