Closes #441

Proposed Changes

This PR allows to use new DateTime(2015, 10, 15, 8, 20, 15, DateTimeKind.Local); or new DateTime(2015, 10, 15, 8, 20, 15, DateTimeKind.Unspecified) as a timestamp for PointData:

var dateTime = new DateTime(2015, 10, 15, 8, 20, 15, DateTimeKind.Unspecified);

var point = PointData.Measurement("h2o")
    .Tag("location", "europe")
    .Field("level", 2)
    .Timestamp(new DateTime(2015, 10, 15, 8, 20, 15, DateTimeKind.Unspecified), WritePrecision.Ms);

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• dotnet test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)