Fix cookies for subdomains setup

[kshnurov]

Due to changes made in #64 it's not possible to disable cookies for subdomains, they're always on.

Expected behavior

1. setDomain('example.com') or init(..., { domain: 'example.com' }); would set cookies for example.com without subdomains
2. setDomain('') or init(..., { domain: '' }); or init(..., { domain: null }); would set cookies for the current domain without subdomains

Actual behavior

1. setDomain('example.com') or init(..., { domain: 'example.com' }); sets cookies for .example.com.
2. setDomain('') or init(..., { domain: '' }); or init(..., { domain: null }); is ignored due to non-empty string check here

This happens because of the document.cookie syntax: leading dots in domain names are ignored, but browsers may decline to set the cookie containing such dots. If a domain is specified, subdomains are always included.

New behavior

1. Docs are updated to reflect the fact that any specified domain will include all subdomains. There's no workaround for that. Trailing dot for cookies domain is useless, may lead to cookies being declined and has been removed.

2. setDomain('') or init(..., { domain: '' }); or init(..., { domain: null }); sets cookies only for the current domain since document.cookie= is called without a specific domain.