Diff fix

I found some behavior are different between _s and python,

 - `[1,2,3,4,5,6][1:-1]` return [2,3,4,5] in python, but in _s return []
 - `[1,2,3,4,5,6][0:]` return [1,2,3,4,5,6] in python, but in _s return [1] 
 - `[1,2,3,4,5,6][-1:1:-1]` return [6,5,4,3,2] in python, but in _s return []  

I don't know it is designed like this or just a bug.

so here are my version of _s fixed these different, and all the test in README.md have the same return.