updated cookie deletion instructions to prevent inconsistent safari behavior

when setting/getting cookies in safari, it behaves a bit differently from chrome/ff/etc. example:

in chrome:

```
var cookie = require('cookie-cutter');
cookie.set('meh', null, { path: '/', expires: new Date(0) });

var meh = cookie.get('meh'); 
```
meh is `undefined` in Chrome. However in Safari, meh is the string `"null"`. subtle but important difference, especially since many people will rely on the "truthiness" of the return result in setting up their logic.

This note provides specific guidance to explicitly use an empty string to avoid this effect.