refacor PR #132 - localStorage.length issue

Actually it's a nice feature, but no offense,  it's bad PR
String.prototype.contains it's a part of ECMA 6 standards, and it's not support basic browsers. 
see: [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/contains)
@ecordoba24 tried to use global localStorage that even not exist in the current scope, so actually this not work.<br/>
one last thing..
I think you should only accept PR that are fully tested and not break the coverage..
(We could add this label: `pr-needs-work`, like other project see: [mocha](https://github.com/visionmedia/mocha) for example)