GH-568 Add support for casting resulting function as Runnable

Resolves #568

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/SimpleFunctionRegistry.java

@@ -415,7 +415,8 @@ public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspect
 	 *
 	 * @author Oleg Zhurakousky
 	 */
-	public class FunctionInvocationWrapper implements Function<Object, Object>, Consumer<Object>, Supplier<Object> {
+	public class FunctionInvocationWrapper implements Function<Object, Object>, Consumer<Object>,
+			Supplier<Object>, Runnable {
 
 		private final Object target;
 
@@ -519,6 +520,11 @@ public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspect
 			return this.doApply(input, false, enricher);
 		}
 
+		@Override
+		public void run() {
+			this.apply(null);
+		}
+
 		public Type getFunctionType() {
 			return this.functionType;
 		}

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/catalog/BeanFactoryAwareFunctionRegistryTests.java

@@ -484,6 +484,13 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		assertThat(f.apply("Bubbles")).isEqualTo("BUBBLES");
 	}
 
+	@Test
+	public void testSupplierConsumerAsRunnable() {
+		FunctionCatalog catalog = this.configureCatalog(SampleFunctionConfiguration.class);
+		Runnable f = catalog.lookup("numberword|imperativeConsumer");
+		f.run();
+	}
+
 	@EnableAutoConfiguration
 	public static class PojoToMessageFunctionCompositionConfiguration {