Fixes#65
```python
def test_specific_precise_if_unncessary(self):
delta = timedelta(hours=2, minutes=40)
result = format_duration(delta, "%02.0H:%02.0m")
self.assertEqual(result, "02:40")
```
This test fails by returning "03:40" instead. The problem is in the way `format_duration` handles fractional hours.
To fix it, we need to switch between using hours and fractional hours
depending on if minutes are present in the formatted string.
Lukáš Kucharczyk