2.5: Fixing Mysql::describe() for timestamp.

CakeTestFixture could not import schema when it has CURRENT_TIMESTAMP as a default value.

For example:
```sql
CREATE TABLE timestamp_default_values (
	id INT(11) NOT NULL AUTO_INCREMENT,
	phone VARCHAR(10),
	limit_date TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
	PRIMARY KEY(id)
);
```

Mysql::createSchema() contains "`limit_date` timestamp DEFAULT NULL NOT NULL".
It is expected "NULL" or "NOT NULL" instead of "DEFAULT NULL NOT NULL".

No, I hesitate nullable or not. I will delete this line and update test case.
thank you.