The MysteryMan avatar is returned when a non-existing user is requested with an empty default option

When an avatar is requested with a malformed default option with no value on a non-existing user the 'mm.png' file is returned.

Example:

curl -i https://avatars.linux-kernel.at/avatar/nobodyasdasdasdasdashdakhdkjhkaa?d=

While not technically against the API this is not the behaviour of both Libravatar 0.1 and Gravatar, where the default 'nobody.png' file is used instead. This is mentioned briefly, although I think not very clearly, in the documentation:

d or default parameter defaults to the Libravatar logo

Issue found with my tests script ivatar.t.