The Domain and Path field of a parsed cookie have been
the unprocessed wire data since Go 1.0; this seems to
be okay for most applications so let's keep it.

Returning the unprocessed wire data makes it easy to
handle nonstandard or even broken clients without
consulting Raw or Unparsed of a cookie.

The RFC 6265 parsing rules for domain and path are
currently buried in net/http/cookiejar but could be
exposed in net/http if necessary.

R=bradfitz, nigeltao
CC=golang-codereviews
https://golang.org/cl/48060043