Http headers may contain duplicate keys
The current implementation for handling http headers is incorrect. Headers may actually contain duplicate keys, while our current implementation would replace duplicate keys in headers. See: 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.