Fix the spec, which was broken by 6dc56da

Since 6dc56da introduced range check for the 7th argument of Time#new,
we need a value that falls in the permitted range that exhibits the
probelem raised by JRUBY-6592. 86399.9 turns out to be an easy and good
choice.

This spec still fails at commit da0f2eb but passes at commit d72c9f4.