Update unprocessableEntity to allow body

Issue: SPR-12515

Author: rstoyanchev

spring-web/src/main/java/org/springframework/http/ResponseEntity.java

@@ -252,7 +252,7 @@ public static HeadersBuilder<?> notFound() {
 	 * @return the created builder
 	 * @since 4.1.3
 	 */
-	public static HeadersBuilder<?> unprocessableEntity() {
+	public static BodyBuilder unprocessableEntity() {
 		return status(HttpStatus.UNPROCESSABLE_ENTITY);
 	}
 

spring-web/src/test/java/org/springframework/http/ResponseEntityTests.java

@@ -122,11 +122,11 @@ public void notFound() throws URISyntaxException {
 
 	@Test
 	public void unprocessableEntity() throws URISyntaxException {
-		ResponseEntity<Void> responseEntity = ResponseEntity.unprocessableEntity().build();
+		ResponseEntity<String> responseEntity = ResponseEntity.unprocessableEntity().body("error");
 
 		assertNotNull(responseEntity);
 		assertEquals(HttpStatus.UNPROCESSABLE_ENTITY, responseEntity.getStatusCode());
-		assertNull(responseEntity.getBody());
+		assertEquals("error", responseEntity.getBody());
 	}
 
 	@Test