Wrong size is returned for default=retro and default=identicon

When an avatar is requested with a default option of 'retro' or 'identicon' on a non-existing user a wrongly sized image is returned. This image will always be 20px large.

Example:

https://avatars.linux-kernel.at/avatar/nobodyasdasdasdasdashdakhdkjhkaa?s=80&d=retro

This image will be 100 pixels wide.

Issue found with my tests script ivatar.t.