WPSC_Purchase_Log->is_transaction_completed() should check against array of filtered completed order statuses

Fix for #1360

Not sure if this is the best approach.

Both methods seem to do a similar job, but one is static and once is not.

Either way, if one method is deprecated the other will need to alias the remaining method.

I have taken a punt that the more recent method is the one which should be used to check order status - the naming of the method is more descriptive anyway.

I guess is_transaction_completed() could stay as there may be checks in future that require testing if the order status is complete and another condition.

Ben