Fix DueDate.StringDate conversion to and from DateTime.

[AhmedZaki99]

Fixes #36

First, existing unit tests for the DueDate model should be modified to run on different time zones to ensure that date conversion works correctly no matter where it runs.
That's why on the first commit, a FakeLocalTimeZone helper class is added to mimic different time zones for tests when necessary (see this SO question for more details), which is used with the DueDate model tests to randomize the time zone in which tests are run.

Second, additional tests are added for the DueDate model to test the date conversion in the StringDate property getter and setter, which fail as expected.

Last, the StringDate to Date conversion process has been modified to use the DateTimeStyles.RoundtripKind with the DateTime.Parse method. Then when converting back, the round-trip format is used instead of the sortable-date format, with the exclusion of milliseconds.

Note: The DueDate floating date assignment unit test has been modified to use a date with DateTimeKind.Unspecified instead of DateTimeKind.Utc since floating dates are treated now as non-UTC dates.

[sonarqubecloud]

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[olsh]

Thank you for your contribution! 👍🏻

[olsh]

@AhmedZaki99
Sorry, but I found an issue with the PR. This test fails with the changes.

todoist-net/src/Todoist.Net.Tests/Services/ItemsServiceTests.cs

Lines 186 to 199 in 2a0bead

	[Trait(Constants.TraitName, Constants.IntegrationFreeTraitValue)]
	public void CreateNewItem_DueDateIsLocal_DueDateNotChanged()
	{
	var client = TodoistClientFactory.Create(_outputHelper);
	var item = new Item("New task") { DueDate = new DueDate(DateTime.Now.AddYears(1).Date) };
	var taskId = client.Items.AddAsync(item).Result;
	var itemInfo = client.Items.GetAsync(taskId).Result;
	Assert.Equal(item.DueDate.Date, itemInfo.Item.DueDate.Date?.ToLocalTime());
	client.Items.DeleteAsync(item.Id).Wait();
	}

I should have run the test before merging the PR, my bad.
I've reverted the PR with the new PR.
#39

Please open a new PR with fixes.
Also, I've reopened the related issue #36