fix DateTime#jisx0301, it was calling DateTime#iso8601 which is not Date#iso8601
stop depending on Ruby version in RubyDateFormat

* nsec will be 0 in 1.8 anyway
use Date arbitrary precision in #strftime (%N,%L) output

* need ThreadContext, use constructor to pass it
* pass all related Date/DateTime tests
* more arguments, but casting from Object (long versus Rational) seems bad