Header maps header keys to values. If the response had multiple headers with the same key, they may be concatenated, with comma delimiters. (Section 4.2 of RFC 2616 requires that multiple headers be semantically equivalent to a comma-delimited sequence.) Values duplicated by other fields in this struct (e.g., ContentLength) are omitted from Header.

Keys in the map are canonicalized (see CanonicalHeaderKey).