Restore method.setAccessible(true) in the LMP

https://build.spring.io/browse/INT-MASTER-444/

Looks like we still need `method.setAccessible(true)` in the `LambdaMessageProcessor` even if method is `public`.

I think the problem is because any Lambda is still **inline class**, so even if method is  `public`, the class isn't

spring-integration-core/src/main/java/org/springframework/integration/handler/LambdaMessageProcessor.java

@@ -77,6 +77,7 @@ public class LambdaMessageProcessor implements MessageProcessor<Object>, BeanFac
 				"classes with single method - functional interface implementations.");
 
 		this.method = methodValue.get();
+		this.method.setAccessible(true);
 		this.parameterTypes = this.method.getParameterTypes();
 		this.payloadType = payloadType != null ? TypeDescriptor.valueOf(payloadType) : null;
 	}