API access URLs should contain the digest type

Christoph Anton Mitterer suggested the following in bug #1160143:

IMHO it could eventually become problematic, that the access URL doesn't contain the digest type, i.e. it's just
http://cdn.libravatar.org/avatar/40f8d096a3777232204cb3f796c577b7?s=100
and not
http://cdn.libravatar.org/avatar/md5:40f8d096a3777232204cb3f796c577b7?s=100
or e.g.
http://cdn.libravatar.org/avatar/40f8d096a3777232204cb3f796c577b7?md=MD5&s=100
or something like that.

Even though it seems unlikely right now, one might in some future distant want to change the hash algo, probably less because of security per se, but rather as MD5/SHA256 might no longer be widespread...
And the size of the hash alone is not enough to tell the type... future ones will also use the "common lengths".