t5551: make the test for extra HTTP headers more robust

To test that extra HTTP headers are passed correctly, t5551 verifies that a fetch succeeds when two required headers are passed, and that the fetch does not succeed when those headers are not passed. However, this test would also succeed if the configuration required only one header. As Apache's configuration is notoriously tricky (this developer frequently requires StackOverflow's help to understand Apache's documentation), especially when still supporting the 2.2 line, let's just really make sure that the test verifies what we want it to verify. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>