fix parsing cookie Domain attribute with leading dot

[mamantoha]

Domain with leading dot (e.g. .example.com) breaks cookies parser.

Code to reproduce the issue:

cookie = "foo=bar; expires=Thu, 01 Jan 1970 00:00:01 GMT; path=/; domain=.example.com; SameSite=None; secure; HttpOnly"

puts HTTP::Cookie::Parser.parse_set_cookie(cookie).inspect

Before:

#<HTTP::Cookie:0x7fce81225ee0 @name="foo", @value="bar", @path="/", @expires=1970-01-01 00:00:01.0 UTC, @domain=nil, @secure=false, @http_only=false, @samesite=nil, @extension="domain=.example.com; SameSite=None; secure; HttpOnly">

After:

#<HTTP::Cookie:0x7f09a3b29ee0 @name="foo", @value="bar", @path="/", @expires=1970-01-01 00:00:01.0 UTC, @domain="example.com", @secure=true, @http_only=true, @samesite=None, @extension=nil>

According to RFC 6265 section 4.1.2.3

(Note that a leading %x2E ("."), if present,
is ignored even though that character is not permitted, but a
trailing %x2E ("."), if present, will cause the user agent to ignore
the attribute.) If the server omits the Domain attribute, the user
agent will return the cookie only to the origin server.

[sdogruyol]

Thank you @mamantoha 🙏