GitHubMilestoneApiTests due_on Should Use LocalDate

[rwinch]

GitHubMilestoneApiTests uses Instant.now() for due_on. Since Instant.now() is UTC time based, isMilestoneDueTodayWhenDueTodayThenTrue fails when the computer that runs the test is not the same day as it is in UTC time.

To fix it, due_on should be set to an Instant based upon the timezone of the current computer.