Drop all container-update overrides on COPY, as that seems more
consistent than selecting just a couple overrides to drop.
Check that all `actual` headers are also `retained_headers` in
test_purge_crypto_sysmeta_headers.
Check that we really did purge the crypto meta in test_GET_success and
test_HEAD_success. GETs were already getting verified over in
test_encrypter_decrypter.py, but they would fail in a less-obvious way.
Change-Id: I7c17fdc02a9895495a1737d4040e868482bb9e98
c944e73d43