[http] Recognise status code 303 as valid

As RFC 2616 10.3.4 explains, a 303 status is the proper HTTP 1.1
behavior for what most HTTP 1.0 clients did with code 302.

Signed-off-by: Jason Lunz <lunz@acm.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpcore.c

 	case 206:
 	case 301:
 	case 302:
+	case 303:
 		return 0;
 	case 404:
 		return -ENOENT;