Option to not remove non-standard header fields from 304 responses

Filter304Headers On : [default] current behavior, only keep
                      the following header fields in a 304 response:
                        * Connection
                        * Keep-Alive
                        * ETag
                        * Content-Location
                        * Expires
                        * Cache-Control
                        * Vary
                        * Warning
                        * WWW-Authenticate
                        * Proxy-Authenticate
                        * Set-Cookie
                        * Set-Cookie2

Filter304Headers Off: treat 304 no different from other responses
                      and pass all header fields through

This is for fastly/Ops#42