Fix to allow Zend_Db_Expr as column default

[scottsb]

Description

Currently if you attempt to use a Zend_Db_Expr as the default value for a column, it fails because it is accidentally converted to a string too early, which results in a malformed SQL query.

This is due to a check that attempts to avoid double quoting but isn't careful to ensure the passed value is a string, triggering an implicit type conversion.

Manual testing scenarios

Create a column in a setup script similar to this:

    /** @var \Magento\Framework\Setup\SchemaSetupInterface $setup */
    $setup->getConnection()->addColumn(
        'test_table',
        'test_column',
        [
            'type' => Table::TYPE_DATETIME,
            'default' => new \Zend_Db_Expr('CURRENT_TIMESTAMP'),
            'comment' => 'Test column comment'
        ]
    );

Before this PR that fails with the exception:

Syntax error or access violation: 1067 Invalid default value for 'test_column'

After this PR the column is created correctly.

[magento-team]

@scottsb thank you for your contribution