fix another test

Author: Sun

src/test/java/com/fasterxml/jackson/module/jsonSchema/TestUnwrapping.java

@@ -28,11 +28,12 @@ static class Name {
 
     public void testUnwrapping()  throws Exception
     {
-        MAPPER.configure(MapperFeature.SORT_PROPERTIES_ALPHABETICALLY, true);
-        JsonSchemaGenerator generator = new JsonSchemaGenerator(MAPPER);
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.configure(MapperFeature.SORT_PROPERTIES_ALPHABETICALLY, true);
+        JsonSchemaGenerator generator = new JsonSchemaGenerator(mapper);
         JsonSchema schema = generator.generateSchema(UnwrappingRoot.class);
 
-        String json = MAPPER.writeValueAsString(schema).replace('"', '\'');
+        String json = mapper.writeValueAsString(schema).replace('"', '\'');
 
         String EXP = "{'type':'object'," +
                      "'id':'urn:jsonschema:com:fasterxml:jackson:module:jsonSchema:TestUnwrapping:UnwrappingRoot'," +