Don't collapse Set-Cookie header values

[fukamachi]

This would be a problem mainly of parsing an HTTP response.

RFC 2109 is saying:

   An origin server may include multiple Set-Cookie headers in a
   response.  Note that an intervening gateway could fold multiple such
   headers into a single header.

However, It is very common an attribute "Expires" contains "," in its value like "Wed, 04 Mar 2015 07:05:42 GMT".

In RFC 6265, which deprecates RFC 2109, Set-Cookie header can have only single Cookie in its value.