Plug.Conn.put_resp_header doesn't allow multiple headers with the same name

[mdg]

According to documentation, put_resp_header() replaces an existing value if one exists.
https://hexdocs.pm/plug/Plug.Conn.html#put_resp_header/3

However, the HTTP RFC 2616 says that:

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)].

This is particularly problematic when trying to add multiple Set-Cookie headers that are already encoded into the cookie format. I've encountered this issue while trying to write a reverse proxy w/ Plug and I'd like to be able to pass the upstream cookies straight into the Plug.Conn object w/o having to parse them and then reset them into the Plug.Conn cookie structure.

[josevalim]

You can manually add the headers to the struct %{conn | resp_headers: set_cookie_headers ++ conn.resp_headers}`. In fact, if you are using a reverse proxy, that's like the simplest way to move all headers over.