Handle default values for boolean, datetime, and datetimetz columns in PostgreSQL

When dealing with legacy schema it would be nice to be able to map default values and not have the schema spit out `ALTER` statements each time. This works correctly today for basic integer and string columns, but does not handle columns with special types such as `boolean` or `datetime`.

For example, the following statement:

`ALTER TABLE test_table ALTER test_column SET DEFAULT CURRENT_TIMESTAMP;`

Results in a column definition like:

`test_column | timestamp with time zone | not null default now()`

However, repeating the same schema generation will result in the same `ALTER` statement each time, because it will always detect that the default value has changed. The same is true for boolean columns.

This simple change prevents this situation from happening and correctly detects that the column default has not changed. It is specific to PostgreSQL.
Isn't this case actually captured [here](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Schema/PostgreSqlSchemaManager.php#L382-L392) already?