The feature is only enabled if the output is believed to be a tty.

-J: There's some minor differences and improvements in -J handling, as
now J should work with -i and it actually creates a file first using the
initial name and then *renames* that to the one found in
Content-Disposition (if any).

-i: only shows headers for HTTP transfers now (as documented).
Previously it would also show for pieces of the transfer that were HTTP
(for example when doing FTP over a HTTP proxy).

-i: now shows trailers as well. Previously they were not shown at all.

--libcurl: the CURLOPT_HEADER is no longer set, as the header output is
now done in the header callback.

are not, due mainly to the lack of support for XML character entities
(e.g. & => & ).  This will make it easier to validate test files using
tools like xmllint, as well as edit and view them using XML tools.

trailer is then sent to the normal header callback/stream.