HTTP header fields, […] follow the same generic format as that given in Section 3.1 of RFC 822 . Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.
and e. g. section 6.2 (and all other sources I checked) list response headers keys in mixed case, what is the point of:
Plug.Conn.InvalidHeaderError) header key is not lowercase: "WWW-Authenticate"
error I receive when testing the functionality, which returns the above header?