Fix Compiled SpEL Test

Several problems:

- Setting the system property is not enough since the `static` parser already has its configuration
- Changing the ParametersWrapper to `private` makes property accessing not compilable (the method and class must be public).
- The expression...

    #target.messageAndHeader(message, headers['number'] != null ? headers['number'] : T(org.springframework.util.Assert).isTrue(false, 'required header not available: number'))

...is not compilable anyway because the right arguent to the `!=` is not compilable because the `TypeReference.exitTypeDescriptor` is `null`.

This commit addresses the first two.

Polishing

Use a compilable method.

* Simple polishing

spring-integration-core/src/main/java/org/springframework/integration/util/MessagingMethodInvokerHelper.java

@@ -785,10 +785,6 @@ public class MessagingMethodInvokerHelper<T> extends AbstractExpressionEvaluator
 			return (T) this.invocableHandlerMethod.invoke(message);
 		}
 
-		public Expression getExpression() {
-			return this.expression;
-		}
-
 		Class<?> getTargetParameterType() {
 			return this.targetParameterType;
 		}
@@ -979,7 +975,7 @@ public class MessagingMethodInvokerHelper<T> extends AbstractExpressionEvaluator
 		}
 	}
 
-	private static class ParametersWrapper {
+	public static class ParametersWrapper {
 
 		private final Object payload;