Preserve class keyword as Map key during update mapping.

This commit makes sure to skip the class property ob Object when mapping maps and their keys inside an Update.

Closes #3566
Original pull request: #3577.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -1220,7 +1220,8 @@ public class QueryMapper {
 
 		private boolean isPathToJavaLangClassProperty(PropertyPath path) {
 
-			if (path.getType().equals(Class.class) && path.getLeafProperty().getOwningType().getType().equals(Class.class)) {
+			if ((path.getType() == Class.class || path.getType().equals(Object.class))
+					&& path.getLeafProperty().getType() == Class.class) {
 				return true;
 			}
 			return false;

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/UpdateMapperUnitTests.java

@@ -1180,6 +1180,17 @@ class UpdateMapperUnitTests {
 				.isEqualTo("{\"$set\": {\"map.601218778970110001827396.value\": \"testing\"}}");
 	}
 
+	@Test // GH-3566
+	void mapsObjectClassPropertyFieldInMapValueTypeAsKey() {
+
+		Update update = new Update().set("map.class", "value");
+		Document mappedUpdate = mapper.getMappedObject(update.getUpdateObject(),
+				context.getPersistentEntity(EntityWithObjectMap.class));
+
+		assertThat(mappedUpdate)
+				.isEqualTo("{\"$set\": {\"map.class\": \"value\"}}");
+	}
+
 	static class DomainTypeWrappingConcreteyTypeHavingListOfInterfaceTypeAttributes {
 		ListModelWrapper concreteTypeWithListAttributeOfInterfaceType;
 	}