DateValue.toRational() fails in year 2038

[jim-easterbrook]

DateValue.toRational() returns the date as a UNIX calendar time converted to Rational. This is a pair of signed int32 so cannot handle the 0x7fffffff 31-bit overflow of the signed UNIX epoch.

This was discovered by someone running tests on my Python interface to exiv2.
jim-easterbrook/python-exiv2#44

(I can't think why anyone would want a UNIX timestamp as a Rational, but the method is there so it ought to work.)