Previously Request and Response had redundant fields for
Referer, UserAgent, and cookies which caused confusion and
bugs.  It also didn't allow us to expand the package over
time, since the way to access fields would be in the Headers
one day and promoted to a field the next day.  That would be
hard to gofix, especially with code ranging over Headers.

After a discussion on the mail package's design with a similar
problem, we've designed to make the Headers be the source of
truth and add accessors instead.

Request:
change: Referer -> Referer()
change: UserAgent -> UserAgent()
change: Cookie -> Cookies()
new: Cookie(name) *Cookie
new: AddCookie(*Cookie)

Response:
change: Cookie -> Cookies()

Cookie:
new: String() string

R=rsc
CC=golang-dev
https://golang.org/cl/4620049