For http2 we have to lowercase, but that can be how we use Header vs a
restriction in its constructor.  Since we have to lowercase anyway, and it
is also measurably faster, I was thinking we ought to use lc comparison...

HTTP header field names are strings of ASCII characters that are compared
in a case-insensitive fashion. Header field names MUST be converted to
lowercase prior to their encoding in HTTP/2.0. A request or response
containing uppercase header field names MUST be treated as malformed
(Section 8.1.3.5).