Keep default=false for boolean columns

A little improvement to a corner case. I have a boolean field with default to false.

In my schema.php I have :
```php
array('type' => 'boolean', 'default' => false)
```
From the database, before this patch it reads :
```php
array('type' => 'boolean')
```
So when I use schema update, I have a change whereas there isn't any.

This PR fixes it.

Hope it makes sense,
Rémi