index_exists() not working due to $column being prepared

[Ashley Gibson]

$column is being prepared here: https://github.com/berlindb/core/blob/master/table.php#L597

That turns the query into this:

SHOW INDEXES FROM wp_edd_orders WHERE 'Key_name' LIKE 'status';

But having Key_name in quotes like that invalidates it. It needs to be:

SHOW INDEXES FROM wp_edd_orders WHERE Key_name LIKE 'status';