Fixes bug in chef_auth_spec.rb

JRuby < 1.7.5 flips the sign in Time.new when passed a UTC offset.
Test pass under JRuby because the wrong sign was used. The tests
should pass in everything now except JRuby < 1.7.5. Hiro Asari has
a fix in place for JRuby 1.7.5, at which point the tests should
pass everywhere.