Modify logic in Convert Method of TimeUtils.java

[drtobster]

Fixes #49

The logic for converting to UTC from a local time and from UTC to a local time in method TimeUtils.convert has been updated. Once the milliseconds from the Epoch has been obtained, those milliseconds and the relevant timezone are passed to method toDateTimeValue(long millisFromEpoch, TimeZone zone) in TimeUtils.java for conversion to a DateTimeValue type. That method converts the milliseconds from the Epoch into a GregorianCalendar type first. This steps takes care of all timezone offsets adjustments as well as handling the DST "gap" and "overlap" hours when applicable.

The implementation in BiWeekly version 0.4.6 did not always return the expected value under certain circumstance. This became apparent when using the advanceTo method on a DateIterator with specific startAt and TimeZone values. See Issue #49 for an example.

The implementation of TimeUtils.convert in versions 0.4.5 and earlier does not contain this problem. However, a DateIterator in those earlier versions does not handle the DST "gap" hour correctly. See Issue #38.

[codecov-io]

Current coverage is 77%

Merging #50 into master will not change coverage

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[mangstadt]

Thank you, Toby. I appreciate the detailed explanation and the fix. This timezone stuff is not easy. :)