GH-588 Fix dropped headers during input conversion

This addresses the issue of dropped Message headers in the event where input is a Message but input conversion is not necessary
while Message contains headers that require explicit propagation (e.g., scf-sink-url, scf-func-name)

Resolves #588

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

@@ -37,6 +37,7 @@ import org.springframework.cloud.function.context.FunctionCatalog;
 import org.springframework.cloud.function.context.FunctionRegistration;
 import org.springframework.cloud.function.context.FunctionRegistry;
 import org.springframework.cloud.function.context.FunctionType;
+import org.springframework.cloud.function.context.HybridFunctionalRegistrationTests.UppercaseFunction;
 import org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry.FunctionInvocationWrapper;
 import org.springframework.cloud.function.context.config.JsonMessageConverter;
 import org.springframework.cloud.function.context.config.NegotiatingMessageConverterWrapper;
@@ -81,6 +82,27 @@ public class SimpleFunctionRegistryTests {
 		this.conversionService = new DefaultConversionService();
 	}
 
+	@SuppressWarnings("unchecked")
+	@Test
+	public void testSCF588() {
+
+		UpperCase function = new UpperCase();
+		FunctionRegistration<UpperCase> registration = new FunctionRegistration<>(
+				function, "foo").type(FunctionType.of(UppercaseFunction.class));
+		SimpleFunctionRegistry catalog = new SimpleFunctionRegistry(this.conversionService, this.messageConverter);
+		catalog.register(registration);
+
+		FunctionInvocationWrapper lookedUpFunction = catalog.lookup("uppercase");
+
+		Message<String> message = MessageBuilder.withPayload("hello")
+				.setHeader("scf-sink-url", "blah")
+				.setHeader("scf-func-name", "blah")
+				.build();
+		Object result = lookedUpFunction.apply(message);
+		assertThat(result).isInstanceOf(Message.class);
+		assertThat(((Message<String>) result).getPayload()).isEqualTo("HELLO");
+	}
+
 	@Test
 	public void testFunctionLookup() {