Ensure gif avatar urls end in .gif

This is a workaround for discord having trouble animating gifs if
the url does not end in exactly `.gif`. Since avatar_url is common
for thumbnails etc., adding this workaround here is handy, and
likely restores expected behavior (animated avatars animating).

discord/user.py

@@ -155,7 +155,10 @@ class BaseUser(_BaseUser):
             else:
                 format = static_format
 
-        return 'https://cdn.discordapp.com/avatars/{0.id}/{0.avatar}.{1}?size={2}'.format(self, format, size)
+        # Discord has trouble animating gifs if the url does not end in `.gif`
+        gif_fix = '&_=.gif' if format == 'gif' else ''
+
+        return 'https://cdn.discordapp.com/avatars/{0.id}/{0.avatar}.{1}?size={2}{3}'.format(self, format, size, gif_fix)
 
     @property
     def default_avatar(self):