♻️ Simplify shutdown of socket in #disconnect

This should work both for TCPSocket and for OpenSSL::SSL::SSLSocket.
Both will respond to `#to_io` with the TCPSocket.

Author: nevans

lib/net/imap.rb

@@ -1139,13 +1139,7 @@ def disconnect
       in_logout_state = try_state_logout?
       return if disconnected?
       begin
-        begin
-          # try to call SSL::SSLSocket#io.
-          @sock.io.shutdown
-        rescue NoMethodError
-          # @sock is not an SSL::SSLSocket.
-          @sock.shutdown
-        end
+        @sock.to_io.shutdown
       rescue Errno::ENOTCONN
         # ignore `Errno::ENOTCONN: Socket is not connected' on some platforms.
       rescue Exception => e