Update cookie.js

Fixed the following bug:
If there is a '=' in the cookie value, for example, the cookie is 'a=a=b',
the cookie.get('a') will return 'a', where the correct return value should be 'a=b'.

Learn the code from jquery.cookie.js, ref https://github.com/carhartl/jquery-cookie/blob/master/jquery.cookie.js