PeriodSerializer not working with midnight DATE-TIME.

[minichma]

PeriodSerializer doesn't deserialize a period's duration correctly if the start IDateTime's time is set to midnight.

E.g. the following period string should be serialized to a period with a duration of 6H but actually has 1D:
20180219T000000Z/PT6H

Can be reproduced via the following test case:

[Test]
public void TestPeriods()
{
    var serializer = new PeriodSerializer();
    var res = serializer.Deserialize(new System.IO.StringReader("20180219T000000Z/PT6H")) as Period;

    Assert.AreEqual(6.0, res.Duration.Hours);
    Assert.IsTrue(res.StartTime.HasTime);
 }

The problem boils down to Period automatically assuming a duration of 1D if the time component is zero.