Enforce en_US_POSIX on date encoding formatter
Enforce en_US_POSIX on date encoding formatter

This enforces en_US_POSIX (24-hour formatting) on the formatter used to encode dates.
When not doing this, i.e. using the current locale, it is possible that 12-hour formatting is used, resulting in an invalid HH component according to RFC3339.

yonaskolb/SwagGen,yonaskolb/SwagGen,yonaskolb/SwagGen