Improve refund email subject/heading string check

Hi there,

WooCommerce Multilingual tries to filter the subject_partial and subject_full (same with heading) for the refund email in:

inc/class-wc-emails.php method filter_refund_emails_strings.

This check is not strong enough as it filters other emails for these particular options/strings as well. E.g. we are using subject_partial and subject_full for our shipment email (https://github.com/vendidero/woocommerce-germanized-shipments/blob/master/includes/emails/class-wc-gzd-email-customer-shipment.php) as well which leads to Woo Multilingual replacing our subject with the refund subject which is not expected behaviour. Please improve your code by checking the email id within filter_refund_emails_strings before replacing.

Cheers