ability to expect-continue only if potentially stale

If the stale check fails, non-idempotent queries such as POST can return a "no response from server" error to the application stack, resulting in ambiguity as to whether the request was received and acted on in some fashion by the host, possibly resulting in a server-side error.

Might be nice as a RequestConfig option to be able to enable the expect-continue handshake only for cases where the connection was recycled from a pool.