Fix parsing of fractional durations

The parsing of fractional durations checked for positivity by testing
`second > 0`, which reports `false` for all fractions between -Infinity
and +1 seconds (exclusive).

The JavaScript libraries that I tested disagree whether "-PT-0,6S"
represents a positive or negative duration, "moment" and "tinyduration"
reported positive, "luxon" (the successor to moment) reported negative.
My reading of the specification gives me no reason to doubt that a
double negative should result in anything but a positive total result.

It is worth mentioning that as the scale units are represented as
integers, there is no clean way to implement fractional parsing of
non-second units. Currently, fractional non-second durations simply fail
to parse. [Wikipedia](https://en.wikipedia.org/wiki/ISO_8601#Durations)
gives "P0.5Y" as an example.

Author: urmastalimaa

Diff for: lib/elixir/lib/calendar/iso.ex

@@ -715,7 +715,15 @@ defmodule Calendar.ISO do
       {second, <<delimiter, _::binary>> = rest} when delimiter in [?., ?,] ->
         case parse_microsecond(rest) do
           {{ms, precision}, "S"} ->
-            ms = if second > 0, do: ms, else: -ms
+            ms =
+              case string do
+                "-" <> _ ->
+                  -ms
+
+                _ ->
+                  ms
+              end
+
             {:ok, [second: second, microsecond: {ms, precision}] ++ acc}
 
           _ ->

Diff for: lib/elixir/test/elixir/calendar/duration_test.exs

@@ -262,6 +262,9 @@ defmodule DurationTest do
     assert Duration.from_iso8601!("PT6S") == %Duration{second: 6}
     assert Duration.from_iso8601!("PT1,6S") == %Duration{second: 1, microsecond: {600_000, 1}}
     assert Duration.from_iso8601!("PT-1.6S") == %Duration{second: -1, microsecond: {-600_000, 1}}
+    assert Duration.from_iso8601!("PT0,6S") == %Duration{second: 0, microsecond: {600_000, 1}}
+    assert Duration.from_iso8601!("PT-0,6S") == %Duration{second: 0, microsecond: {-600_000, 1}}
+    assert Duration.from_iso8601!("-PT-0,6S") == %Duration{second: 0, microsecond: {600_000, 1}}
     assert Duration.from_iso8601!("-P10DT4H") == %Duration{day: -10, hour: -4}
     assert Duration.from_iso8601!("-P10DT-4H") == %Duration{day: -10, hour: 4}
     assert Duration.from_iso8601!("P-10D") == %Duration{day: -10}