GH-243, GH-257 Added reactive consumer wrapper

- Added wrapper for an already reactive consumer to ensure that consumers can be consistently represented as Function<Flux, Mono>
- Fixed the big that deal with inconsistent result in web environments due to inconsistent representation of the Consumers
- Polished tests

Resolves #243
Resolves #257

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

@@ -560,10 +560,10 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 				"spring.cloud.function.compile.foos.lambda=f -> f.subscribe("
 						+ getClass().getName() + "::set)",
 				"spring.cloud.function.compile.foos.type=consumer");
-		assertThat((Consumer<?>) this.catalog.lookup(Consumer.class, "foos"))
-				.isInstanceOf(Consumer.class);
+		assertThat((Function<?, ?>) this.catalog.lookup(Function.class, "foos"))
+				.isInstanceOf(Function.class);
 		assertThat(this.inspector
-				.getInputWrapper(this.catalog.lookup(Consumer.class, "foos")))
+				.getInputWrapper(this.catalog.lookup(Function.class, "foos")))
 						.isEqualTo(Flux.class);
 		@SuppressWarnings("unchecked")
 		Consumer<Flux<String>> consumer = (Consumer<Flux<String>>) this.context