BUG/MINOR: sample: zero-pad the fractional part emitted by http_date()

BUG/MINOR: sample: zero-pad the fractional part emitted by http_date()

When http_date() is given the "ms" or "us" unit, it appends the
sub-second part of the date with a plain "%d", so it is printed without
any padding: 123 ms gives ".123" but 23 ms gives ".23", which stands
for a completely different instant, and 23 us gives ".23" as well
although the two units must not produce the same string.

The fraction is now printed over the number of digits its unit
requires, three for milliseconds and six for microseconds. The test
which selects the format is also made to depend on that width rather
than on the unit argument, which is equivalent but clearer.

The bug was introduced in 2.1-dev4 with commit ae6f125c7b ("MINOR:
sample: add us/ms support to date/http_date"), so this fix should
be backported to all stable branches.