[http] Fix HTTP SAN booting

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>

src/net/tcp/httpcore.c

 	 * force an error.
 	 */
 	if ( ( http->rx_state < HTTP_RX_DATA ) || ( http->chunked != 0 ) ) {
-		DBGC ( http, "HTTP %p connection closed unexpectedly\n",
-		       http );
+		DBGC ( http, "HTTP %p connection closed unexpectedly in state "
+		       "%d\n", http, http->rx_state );
 		http_close ( http, -ECONNRESET );
 		return;
 	}
 		return -EINVAL_RESPONSE;
 	http->code = strtoul ( spc, NULL, 10 );
 
-	/* Move to received headers */
-	http->rx_state = HTTP_RX_HEADER;
+	/* Move to receive headers */
+	http->rx_state = ( ( http->flags & HTTP_HEAD_ONLY ) ?
+			   HTTP_RX_TRAILER : HTTP_RX_HEADER );
 	return 0;
 }
 
 		}
 
 		/* Move to next state */
-		if ( ( http->rx_state == HTTP_RX_HEADER ) &&
-		     ( ! ( http->flags & HTTP_HEAD_ONLY ) ) ) {
+		if ( http->rx_state == HTTP_RX_HEADER ) {
 			DBGC ( http, "HTTP %p start of data\n", http );
 			http->rx_state = ( http->chunked ?
 					   HTTP_RX_CHUNK_LEN : HTTP_RX_DATA );