Previously it normalized duration in order to expose a more
human-friendly string. For example, 7 days would become a week,
12 months would become a year. It seems nice but might create a
false impression that it is possible to retrieve weeks and years
from an `IsoDuration` object. However, it only supports months,
days, seconds and nanoseconds. Also, other drivers do not normalize
durations.
This commit makes `IsoDuration#toString()` simply concatenate all
values into a string as is. Resulting string will only contain months,
days, seconds and nanoseconds. Zero values are not filtered out. So
returned ISO strings represent exactly same durations but in different
format.