GH-475 Fix support for Flux<List> type conversion

At the moment support is rudimentary but given that BeanFactoryAwareFunctionRegistry provides all the necessary type conversion functionality the true fix should consider utilizing it.

Resolves #475

spring-cloud-function-web/src/main/java/org/springframework/cloud/function/web/RequestProcessor.java

@@ -145,7 +145,7 @@ public class RequestProcessor {
 				Type jsonType = body.startsWith("[")
 						&& Collection.class.isAssignableFrom(inputType)
 						|| body.startsWith("{") ? inputType : Collection.class;
-				if (body.startsWith("[")) {
+				if (body.startsWith("[") && itemType instanceof Class) {
 					jsonType = ResolvableType.forClassWithGenerics((Class<?>) jsonType,
 							(Class<?>) itemType).getType();
 				}

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/flux/HttpPostIntegrationTests.java

@@ -351,6 +351,15 @@ public class HttpPostIntegrationTests {
 				String.class).getBody()).isEqualTo("{\"A\":2,\"B\":1}");
 	}
 
+	@Test
+	public void fluxWithList() throws Exception {
+		List<String> list = Arrays.asList("A", "B", "A");
+		assertThat(this.rest.exchange(
+				RequestEntity.post(new URI("/fluxCollectionEcho")).accept(MediaType.APPLICATION_JSON)
+						.contentType(MediaType.APPLICATION_JSON).body(list),
+				String.class).getBody()).isEqualTo("[\"A\",\"B\",\"A\"]");
+	}
+
 	private String sse(String... values) {
 		return "data:" + StringUtils.arrayToDelimitedString(values, "\n\ndata:") + "\n\n";
 	}
@@ -481,6 +490,11 @@ public class HttpPostIntegrationTests {
 					(map, word) -> map.merge(word, 1, Integer::sum));
 		}
 
+		@Bean
+		public Function<Flux<List<String>>, Flux<String>> fluxCollectionEcho() {
+			return flux -> flux.flatMap(v -> Flux.fromIterable(v));
+		}
+
 	}
 
 	public static class Foo {