JIRA: https://jira.spring.io/browse/INT-3796
Previously, the `DefaultSftpSessionFactory` unconditionally accepted keys
from unknown hosts, or when a host key changed.
Permit this behavior to be configured with `allowUnknownHosts` and/or a custom
`UserInfo` properties in the factory.
`allowUknownHosts` is false by default (since 4.2) and true by default in 4.1.x for
backwards compatibility.
INT-3796: Fix Tests
INT-3796: Polishing
95a9e0d2dd