The fix to enable changing type of a column

Ran into bug with a migration.
Steps to reproduce.
First was:
```php
public function change()
{
	$table = $this->table('user_properties', ['id' => true, 'primary_key' => ['id']]);
	$table->addColumn('user_id', 'integer')
		->addColumn('name', 'string', ['limit' => 20])
		->addColumn('sex', 'integer')
		->save();

	$table = $this->table('user_properties');
	$table
		->addColumn('city', 'string', ['limit' => 50, 'default' => null, 'null' => true])
		->addColumn('birth', 'timestamp', ['default' => null, 'null' => true])
		->update();
}
```
Then:
```php
public function change()
{
	$table = $this->table('user_properties');
	$table
		->changeColumn('birth', 'date', ['default' => '1930-01-01'])
		->update();
	}
```
The error:
```
 [PDOException]                                                              
  SQLSTATE[42601]: Syntax error: 7 ERROR:  syntax error at or near "NULL"     
  LINE 1: ...."user_properties" ALTER COLUMN "birth" TYPE DATE NULL DEFAU...
```