Fix multi-part form handling for WebFlux apps

In WebFlux (but not MVC) you have to explicitly ask for the multi-
part content separately from the form data.

Fixes gh-223

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

@@ -302,7 +302,6 @@ public class HttpPostIntegrationTests {
 	}
 
 	@Test
-	@Ignore
 	public void multipart() throws Exception {
 
 		LinkedMultiValueMap<String, String> map = new LinkedMultiValueMap<>();

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

@@ -307,6 +307,20 @@ public class HttpPostIntegrationTests {
 				String.class).getBody()).isEqualTo("[{\"A\":6,\"B\":11}]");
 	}
 
+	@Test
+	public void multipart() throws Exception {
+
+		LinkedMultiValueMap<String, String> map = new LinkedMultiValueMap<>();
+
+		map.put("A", Arrays.asList("1", "2", "3"));
+		map.put("B", Arrays.asList("5", "6"));
+
+		assertThat(rest.exchange(
+				RequestEntity.post(new URI("/sum")).accept(MediaType.APPLICATION_JSON)
+						.contentType(MediaType.MULTIPART_FORM_DATA).body(map),
+				String.class).getBody()).isEqualTo("[{\"A\":6,\"B\":11}]");
+	}
+
 	@Test
 	public void count() throws Exception {
 		List<String> list = Arrays.asList("A", "B", "A");