Rails has incorrectly been adding leading dots to cookie domain values
when the `domain: :all` option is present.

This leading dot was required in cookies based on [RFC 2965][rfc2965]
(October 2000), but [RFC 6265][rfc6265] (April 2011) changed that
behaviour, making a leading dot strictly incorrect. Todays browsers aim
to confirm to RFC6265 with repect to cookies.

The new behaviour is that *any* cookie with an explicitly passed domain
is sent to all matching subdomains[[ref][mdn]]. For a server to indicate
that only the exact origin server should receive the cookie, it should
instead pass *no* domain attribute.

Despite the change in behaviour, browser devtools often display a cookie
domain with a leading dot to indicate that it is valid for subdomains -
this prefixed domain is *not* necessarily the raw value that was passed
in the Set-Cookie header. This explains why it's a common belief among
developers that the leading dot is required.

RFC6265 standard gives UAs an algorithm to handle old-style cookie
domain parameters (they can drop a leading dot if present), so it's
unlikely that this error would ever have had any effect on web browsers.

However, cookies generated this way can't be processed by Ruby's own
CGI::Cookie class:

> CGI::Cookie.new "domain" => ".foo.bar", "name" => "foo"
ArgumentError: invalid domain: ".foo.bar"

Newer versions of the Ruby CGI library accomodate the same fallback
behaviour (dropping the extra dot) but this isn't a justification for it
being the right way to set a cookie.

[mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies#domain_attribute
[rfc2965]: https://www.rfc-editor.org/rfc/rfc2965#section-3.2
[rfc6265]: https://www.rfc-editor.org/rfc/rfc6265#section-4.1.1