Support multiple http headers with the same name in both directions

[simonmittag]

RFC2616, Section 4.2: https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

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)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

• rewrite downstream -> upstream with multiple headers, comma separated, in order.
• rewrite upstream -> downstream with multiple headers, comma separated, in order.
• exclude Content-Length from rewrite in both directions, but set it explicitly.