false.. ..false and nil..nil

[tompng]

nil..nil nil..false false..nil false..false was all displayed ..

After this fix

$ ruby -Ilib -e binding.irb
irb(main):001> nil..false
=> ..false
irb(main):002> false..nil
=> false..
irb(main):003> false..false
=> false..false
irb(main):004> nil..nil
=> nil..nil
irb(main):005> 

Distinguishing nil..nil from nil..false will make it easy to debug when I mistakenly created wrong range.

[nobu]

It doesn't seem to need to repeat the same comparisons.