I'll give this a proper review in the next day or so (busy at PyCon).

Despite the fact that I typically hate changing values returned by the server, I agree on case-folding the fact names to lowercase upon reading them. The RFC clearly states they're case insensitive: "Fact names are case-insensitive.  Size, size, SIZE, and SiZe are the same fact." You should have the facts in MLSD_DATA be mixed case, to ensure they're being lowercased correctly.

I agree that there's nothing that can be done in the case where no facts are specified: the spec clearly says just use the values we've previously set. Caller beware.