Commit 501527d ("[http] Treat any unexpected connection close as an
error") introduced a regression causing HTTP SAN booting to fail.  At
the end of the response to the HEAD request, the call to http_done()
would erroneously believe that the server had disconnected in the
middle of the HTTP headers.

Fix by treating the header block from a HEAD request as a trailer
block.  This fixes the problem and also simplifies the logic in
http_rx_header().

Reported-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>
Signed-off-by: Michael Brown <mcb30@ipxe.org>