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".

Are you sure you can assume that based on the default value being current timestamp, that the field is nullable?