fix dueDate property in TaskQuery

public interface TaskQuery {
...
String PROPERTY_DUEDATE = "dueDate";

while

public class TaskImpl ... {
...
protected Date duedate;

This bug points out that the query API does not yet have enough test coverage. So before fixing this type, a test should be created that fails. And then fixing the typo should make the test succeed.

Also for the other properties and methods, tests should be written. This issue includes checking and fixing that kind of coverage for all other queries in the api.