Encode string values as strings

When a cookie is read that contains a string constisting of digits only, the call of `JSON.parse` will parse the String as a Number. If the number is larger than the JavaScript Number type the returned value is interpolated.

Example:
```javascript
ipCookie('example', '545654545664564564564554554545');
console.log(ipCookie('example')); // 5.4565454566456454e+29
```