Normalize the woocommerce_email_attachments filter return type

In the WC_Email::get_attachments() the return is a array: https://github.com/woothemes/woocommerce/blob/master/includes/abstracts/abstract-wc-email.php#L240-L248

I believe it is good to normalize it and everyone working with array.
Array is better and simpler to work with the filter and add new attachments.