Force RFC-822 date compliant on any default locale

java.text.SimpleDateFormat uses the system's default locale if not
specified. It may cause this library to output date-time strings
that do not comply with RFC-822 date-time format on systems in
which the default locale is not English.
Merge pull request #11 from builtinnya/force-rfc822

Force RFC-822 date compliant on any default locale