HTTP/3: restricted symbols in header names. As per HTTP/3 draft 27, a request or response containing uppercase header field names MUST be treated as malformed. Also, existing rules applied when parsing HTTP/1 header names are also applied to HTTP/3 header names: - null character is not allowed - underscore character may or may not be treated as invalid depending on the value of "underscores_in_headers" - all non-alphanumeric characters with the exception of '-' are treated as invalid Also, the r->locase_header field is now filled while parsing an HTTP/3 header. Error logging for invalid headers is fixed as well.