Don't compare lowercase versions of strings in naturalCmp

[megawac]

This is pretty inconsistent with other implementations such as PHPs and python's natsort

(I'm using a fork of your lib while these issues are being resolved)

[esamattis]

I'd like it be consistent with the others too but I wonder if this is a bug or feature. Meaning do we need major version release if we change this.

[stoeffel]

Yes I think we need a major release for this. Maybe someone was relaying on array.sort(_.str.naturalCmp) to not change the order of strings with only different capitalisation.

[stoeffel]

After a quick search,I found only one example where someone was using naturalCmp in a case insensitive sort.

https://github.com/NicoleRauch/NodeTestingTechniques/blob/202535d31fea98a3cfcf843f9754a3b998ff2ec0/sources/HTMLtests/problem/lib/members/memberstore.js

var _s = require('underscore.string');

var sortCaseInsensitive = function (objectlist) {
  return objectlist.sort(function (a, b) {
    return _s.naturalCmp(a.lastname + ' ' + a.firstname, b.lastname + ' ' + b.firstname);
  });

};