t5551: make the test for extra HTTP headers more robust

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>