Fix issue when resigning requests

More fallout from reusing the same request object to resign
a request.  Before setting a value for a header, we need to first
check if the key exists and delete it first, otherwise it's an
append, not a replace, which will cause signing errors.

Though not strictly required for some signers, I think it's important
to be consistent and just state that before setting a header value,
we should delete it first if it exists.