always pass four digit years to Time::Local::timelocal

[plicease]

This is for #5

This change makes sure that four digit years are passed to timelocal unmolested. It was sort of doing the right thing by adding then subtracting 1900 to a year that already had four digits in it, but it is pretty confusing.

I am not changing the (intended) behavior of treating 9x and 0-89 as 199x and 20xx respectively. They are converted to four digit years before being passed into timelocal so we don't have to worry about the timelocal bug where the current year changes its interpretation of two digit years. Long term this interpretation is probably wrong, however, I don't have any examples where apache provides a two digit year, so the point may be moot, and in any-case I can't accurate test for the correct behavior. I've added appropriate comments in the code so the maintainer in 2090 can at least understand why I punted on this. Sorry!

I've included tests for four digit years between 1970 and 2500.