plain: don't append charset for binary MIME types

When outputting the Content-Type HTTP header we print the MIME type and
then append "; charset=<charset>" if the charset variable is non-null.

We don't want a charset when we have selected "application/octet-stream"
or when the user has specified a custom MIME type, since they may have
specified their own charset.  To avoid this, make sure we set the page's
charset to NULL in ui-plain before we generate the HTTP headers.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>