According to wikipedia its possible for the fractional separator to be either a comma or a dot.
I noticed this issue when I tried to parse the output of date:
$ date --iso-8601=ns
This pullrequest includes a test and fix for the issue in the case of fractions for seconds.
However, I do believe iso8601 (at least according to wikipedia) supports decimal factions for hours and minutes as well, which does not seem to be supported currently by this library. Should I create another issue for those cases?