Use preferred representation for mail body time stamp

When formatting the time stamp present in the test message body, use
the preferred date and time representation for the current locale.

Tested against:

  - ja_JP.utf8
  - ko_KR.utf8
  - en_US.utf8
  - ru_UA.utf8

Also, it's MSWin32, not MsWin32 (See perlport(1))