add support for the 303 response code.

according to the spec I found the new location SHOULD be retrieved using GET;
we will do this correctly because postbody is not set, as far as I could see.
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html)

Reference: /n/sources/patch/applied/webfs-303
Date: Thu Nov  9 15:14:12 CET 2006
Signed-off-by: Axel.Belinfante@cs.utwente.nl

--- /sys/src/cmd/webfs/http.c	Thu Nov  9 15:07:54 2006
+++ /sys/src/cmd/webfs/http.c	Thu Nov  9 15:07:51 2006
@@ -359,6 +359,7 @@
 
 	case 301:	/* Moved Permanently */
 	case 302:	/* Moved Temporarily */
+	case 303:	/* See Other */
 	case 307: /* Temporary Redirect  */
 		redirect = 1;
 		break;