Issue with duplicate headers in response.

[Harper04]

Hi,
i would like to point an issue i found while proxying for example keycloak.
Keycloak uses the "set-cookie"-Header multiple times.
In streaming mode reverse proxy plug does not pass on these headers because it is reducing on the response headers using Conn.put_resp_header which will update existing headers.
In buffering mode it works as there it is just passed on.

[mwhitworth]

I think there's a solution in #152.

I'll adapt this and provide a regression test for this new functionality (to identify the case described) above and make a release shortly

[mwhitworth]

#168 address this issue