Set-Cookie headers do not get combined in 1.16

Created by: tomlagier

Minor version bump of http-proxy caused our Ember-cli app to only forward a single cookie from the back-end, rather than all cookies. I believe this is because of the rewrite to web-outgoing.js/writeHeaders.

 Set-Cookie: GOP_LOGIN=1001; Path=/; Domain=localhost
 Set-Cookie: GOP_ACCOUNT=1001; Path=/; Domain=localhost
 Set-Cookie: PLAY_SESSION="xxx"; Path=/

Gets turned into this in the response header:

Set-Cookie:GOP_ACCOUNT=1001; Path=/; Domain=localhost