Allow Shell.input() to be overridden in subclasses

- the ssh-shell-spring-boot extensions needs to extend to override the Shell.input()
  method to handle the input provided via the SSH session properly

spring-shell-core/src/main/java/org/springframework/shell/Shell.java

@@ -190,7 +190,7 @@ public class Shell {
 	 * result
 	 * </p>
 	 */
-	private Object evaluate(Input input) {
+	protected Object evaluate(Input input) {
 		if (noInput(input)) {
 			return NO_INPUT;
 		}