Default avatar not working

[Glennmen]

When the user hasn't set a default avatar and you are trying to get their default discord avatar using member.defaultAvatar this returns an invalid URL.

Root cause: PR #401
This PR converted from by default using PNG for all avatars to default using WEBP. This is a good change in general and should remain like this for custom avatars.

But for some reason Discord decided not to make WEBP variants for their default avatars, as shown here:
https://discord.com/developers/docs/reference#image-formatting-cdn-endpoints

Solution:
For defaultAvatar an exception has to be made so that it always returns PNG.