fix : don't use JSON.stringify on primitives

When a string is stored, quote are added around in localStorage cause we use JSON.stringify on it, so when you try to get value without using this library you get wrong stuff.
I had to rewrite get/set part cause I was enabled to test everything properly but it breaks the way you designed tests... I couldn't add a test for undefined value cause it returns me an undefined string not undefined value, I have to see where it comes from but it was not from my modifications, I started tests before doing any modifications.