Make several RestClient::Request methods private.
Make several RestClient::Request methods private.

Make Request#transmit private. It doesn't make any sense as part of the
public API, and I think should never have been public.

Also make setup_credentials, fetch_body, print_verify_callback_warnings,
and process_result private.

acrogenesis-lab/rest-client,ab/rest-client,rest-client/rest-client