RFC 6265 requires that user agents MUST NOT send more than
one Cookie header in a request.

Note, this change also fixes an issue when sending requests
with more than one cookie header line to a php script served
by an apache web server.  Apache concatenates the cookies
with ", ", but php tries to split them only at ";".  E.g.
two cookies: "a=b, c=d" are seen by php as one cookie "a"
with the value "b, c=d".

Fixes #1801

R=bradfitz
CC=golang-dev
https://golang.org/cl/4535048