xnor xnor xnor (aspire beatmap) fails to parse with "cannot convert nan to integer"

[tybug]

from slider import *
library = Library(".")
beatmap = library.lookup_by_id(2628991, download=True)

Traceback (most recent call last):
  File "/Users/tybug/Desktop/coding/osu/slider/slider/library.py", line 256, in lookup_by_id
    return self._read_beatmap(self, beatmap_id=beatmap_id)
  File "/Users/tybug/Desktop/coding/osu/slider/slider/library.py", line 224, in _raw_read_beatmap
    raise KeyError(key)
KeyError: 2628991

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/tybug/Desktop/coding/sandbox/__random.py", line 3, in <module>
    beatmap = library.lookup_by_id(2628991, download=True)
  File "/Users/tybug/Desktop/coding/osu/slider/slider/library.py", line 260, in lookup_by_id
    return self.download(beatmap_id, save=save)
  File "/Users/tybug/Desktop/coding/osu/slider/slider/library.py", line 408, in download
    beatmap = Beatmap.parse(data.decode('utf-8-sig'))
  File "/Users/tybug/Desktop/coding/osu/slider/slider/beatmap.py", line 2102, in parse
    groups['HitObjects'],
  File "/Users/tybug/Desktop/coding/osu/slider/slider/beatmap.py", line 390, in parse
    return parse(Position(x, y), time, hitsound, rest)
  File "/Users/tybug/Desktop/coding/osu/slider/slider/beatmap.py", line 698, in _parse
    duration = timedelta(milliseconds=int(num_beats * ms_per_beat))
ValueError: cannot convert float NaN to integer

Possibly the same underlying cause as #83, I haven't investigated yet. Opening a new issue out of caution in case they are different bugs.