Alter LocalDate.toDateTime to be more consistent

Alter LocalDate.toDateTime(LocalTime)
Alter LocalDate.toDateTime(LocalTime, DateTimeZone)
Document/test equivalent in LocalDateTime

A null LocalTime is delegated to ensure no change in behaviour
Otherwise daylight saving gap and overlap has changed
During DST gap changed from IllegalFieldValueEx to IllegalInstantEx
During DST overlap it now selected the earlier offset

Change avoids use of currentTimeMillis() and avoids issue #177
Fixes #177