Avoid possible NPE in JsonFieldPaths

See gh-715

Author: wilkinsona

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/JsonFieldPaths.java

@@ -71,7 +71,7 @@ private static void from(Set<String> paths, String parent, Object object) {
 		else if (object instanceof Map) {
 			from(paths, parent, (Map<?, ?>) object);
 		}
-		else if (object.equals(ExtractedField.ABSENT)) {
+		else if (ExtractedField.ABSENT.equals(object)) {
 			paths.add(parent);
 		}
 	}

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/JsonFieldPathsTests.java

@@ -35,8 +35,9 @@ public class JsonFieldPathsTests {
 
 	@Test
 	public void noUncommonPathsForSingleItem() {
-		assertThat(JsonFieldPaths.from(Arrays.asList(json("{\"a\": 1, \"b\": [ { \"c\": 2}, {\"c\": 3} ]}")))
-				.getUncommon()).isEmpty();
+		assertThat(
+				JsonFieldPaths.from(Arrays.asList(json("{\"a\": 1, \"b\": [ { \"c\": 2}, {\"c\": 3}, {\"c\": null}]}")))
+						.getUncommon()).isEmpty();
 	}
 
 	@Test