The headers set with --header and --header-download are added to
every request by the rclone transport, including redirect hops which
net/http makes to other hosts, so a credential passed with --header
for one host could be sent to any host that server chose to redirect
to.
The transport now walks the redirect chain net/http records on each
redirected request and, once the chain has visited a host other than
the one originally requested, removes the headers rather than adding
them.
Also restore the global --client-cert and --client-key config after
TestCertificates so its temporary files are not used by later tests.