Add support for composing function of Message with plain function

Fixes gh-267 at least for the most common cases.

spring-cloud-function-context/pom.xml

@@ -53,6 +53,11 @@
 			<artifactId>spring-cloud-function-compiler</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>io.projectreactor</groupId>
+			<artifactId>reactor-test</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 	<build>
 		<plugins>