Support milliseconds when constructing from a Date

[prowe]

Issue #, if available: #725

This pull request modifies the constructor behavior to correctly handle a Date with non-zero milliseconds.

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[popematt]

Thanks for finding and fixing this issue! It looks good, so after the ion-test-driver workflow finishes up (assuming there's no issues found), I'll go ahead and approve and merge this.

[popematt]

FYI, @prowe, the ion-test-driver failure is unrelated to your changes. (Probably caused by amazon-ion/ion-test-driver#24.) I'm going to look into it a bit, but if I can't resolve it soon, I think it's safe to merge your change even though we don't have all the usual test coverage.