Plug.Conn – Erroneous validation of HTTP header keys?

Given that HTTP/1.1 RFC says:

HTTP header fields, […] follow the same generic format as that given in Section 3.1 of RFC 822 [9]. 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?

The comment in the documentation for Plug.Conn.put_req_header sounds VERY relevant:

Thank you for pointing this out. Now I know where it comes from. Apparently I was so baffled that I didn’t even think this could be a documented “feature”.