rgw/civetweb: handle old clients with transfer-encoding: chunked.

rgw/civetweb: handle old clients with transfer-encoding: chunked.

s3 clients *should* provide an x-amz-decoded-content-length field
when they use transport-encoding: chunked.  Some clients do not.
With swift we already allow chunked uploads that do not specify the
content length in advance.   This commit adds similar support
for s3.  Known client affected by this: boto2.

Fixes: https://tracker.ceph.com/issues/45789
Resolves: rhbz#2152801

Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit 22e9509655ca1d799df2eec535390d20f2f60256)