ENH/BUG: update default threshold for _compute_rank

@agramfort @Eric89GXL @mluessi FYI
Because of the way it is worded (or maybe it's just me) My first impression was that there is a default dict which will be used if I supply dict in quotes and a default array which will be used if I supply array in quotes. Maybe it's because you say "pre-specified norms" or maybe it's because I am expecting mne-python to do everything for me ;) Anyway, it's a second order thing :)