Cache result of isMessage() computation before function is wrapped

Otherwise you get the wrong result when you go back and ask later
(but only if the bean is a singleton with no bean definition).

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/ContextFunctionCatalogAutoConfigurationTests.java

@@ -233,6 +233,18 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 				.isAssignableFrom(Integer.class);
 	}
 
+	@Test
+	public void singletonMessageFunction() {
+		create(SingletonMessageConfiguration.class);
+		assertThat(context.getBean("function")).isInstanceOf(Function.class);
+		assertThat(catalog.lookupFunction("function")).isInstanceOf(Function.class);
+		assertThat(inspector.getInputType(catalog.lookupFunction("function")))
+				.isAssignableFrom(Integer.class);
+		assertThat(inspector.getInputWrapper(catalog.lookupFunction("function")))
+				.isAssignableFrom(Integer.class);
+		assertThat(inspector.isMessage(catalog.lookupFunction("function"))).isTrue();
+	}
+
 	@Test
 	public void nonParametericTypeFunction() {
 		create(NonParametricTypeSingletonConfiguration.class);
@@ -529,6 +541,17 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 		}
 	}
 
+	@EnableAutoConfiguration
+	@Configuration
+	protected static class SingletonMessageConfiguration implements BeanFactoryPostProcessor {
+
+		@Override
+		public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)
+				throws BeansException {
+			beanFactory.registerSingleton("function", new SingletonMessageFunction());
+		}
+	}
+
 	@EnableAutoConfiguration
 	@Configuration
 	protected static class NonParametricTypeSingletonConfiguration {
@@ -547,6 +570,15 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 
 	}
 
+	protected static class SingletonMessageFunction implements Function<Message<Integer>, Message<String>> {
+
+		@Override
+		public Message<String> apply(Message<Integer> input) {
+			return MessageBuilder.withPayload("value=" + input.getPayload()).build();
+		}
+
+	}
+
 	@EnableAutoConfiguration
 	@Configuration
 	@ComponentScan(basePackageClasses = GenericFunction.class)