Minor fix related to order status in WC 2.2

Sometimes conflict with 'pending' post_status of WordPress core

Steps to reproduce the issue:

1. Open any order with order status 'pending'
2. Check post_status of this order in wp_posts table, it'll be 'wc-pending'
3. Now without any change, save the same order using 'Save Order' button
4. Now check post_status in table again, it'll be 'pending' instead of 'wc-pending'