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
Was only breaking on some dashboard builds because
not all run the network tests.
R=bradfitzgo, bradfitzwork
CC=golang-dev
https://golang.org/cl/4240086
