fix spelling of capibilities
move auth_capabilities to smtp_client

This allows the client connection to maintain the
auth_capabilities when it is returned to and reused from the pool.
don't mark a connection as unauthenticated when returning to pool

The connection is still active and authenticated at this point,
so no need to mark it as unauthenticated. And attempting to
authenticate again on an already-authenticated session causes
mail servers to gripe.
Merge pull request #307 from toddsf/fix_auth_capabilities

Fix auth capabilities