class Net::SSH::Transport::Session

def select_host_key_verifier(verifier)

[#595](https://github.com/net-ssh/net-ssh/pull/595)
Values false, true, and :very were deprecated in

it is returned directly. Otherwise, an exception is raised.
If the argument happens to respond to :verify and :verify_signature,

- :always (secure)
- :accept_new (insecure)
- :accept_new_or_local_tunnel (insecure)
- :never (very insecure)

a verifier, like `::Net::SSH::Verifiers::Never`.
Usually, the argument is a symbol like `:never` which corresponds to

the parameter.
Instantiates a new host-key verification class, based on the value of 
def select_host_key_verifier(verifier)
  case verifier
  when false
    Kernel.warn('verify_host_key: false is deprecated, use :never')
    Net::SSH::Verifiers::Never.new
  when :never then
    Net::SSH::Verifiers::Never.new
  when true
    Kernel.warn('verify_host_key: true is deprecated, use :accept_new_or_local_tunnel')
    Net::SSH::Verifiers::AcceptNewOrLocalTunnel.new
  when :accept_new_or_local_tunnel, nil then
    Net::SSH::Verifiers::AcceptNewOrLocalTunnel.new
  when :very
    Kernel.warn('verify_host_key: :very is deprecated, use :accept_new')
    Net::SSH::Verifiers::AcceptNew.new
  when :accept_new then
    Net::SSH::Verifiers::AcceptNew.new
  when :secure then
    Kernel.warn('verify_host_key: :secure is deprecated, use :always')
    Net::SSH::Verifiers::Always.new
  when :always then
    Net::SSH::Verifiers::Always.new
  else
    if verifier.respond_to?(:verify)
      if verifier.respond_to?(:verify_signature)
        verifier
      else
        Kernel.warn("Warning: verifier without :verify_signature is deprecated")
        CompatibleVerifier.new(verifier)
      end
    else
      raise(
        ArgumentError,
        "Invalid argument to :verify_host_key (or deprecated " \
        ":paranoid): #{verifier.inspect}"
      )
    end
  end
end