Support plain boolean without option annotations

- Fix so that we can have `boolean arg` without using
  `@Option` or `@ShellOption` defaulting to false.
- Fixes #744

spring-shell-standard/src/main/java/org/springframework/shell/standard/StandardMethodTargetRegistrar.java

@@ -201,6 +201,8 @@ public class StandardMethodTargetRegistrar implements MethodTargetRegistrar {
 								.position(mp.getParameterIndex());
 							if (ClassUtils.isAssignable(boolean.class, parameterType)) {
 								optionSpec.arity(OptionArity.ZERO_OR_ONE);
+								optionSpec.required(false);
+								optionSpec.defaultValue("false");
 							}
 							else if (ClassUtils.isAssignable(Boolean.class, parameterType)) {
 								optionSpec.arity(OptionArity.ZERO_OR_ONE);