Fix: Make sure customer data is properly initialized after WordPress core has
finished initializing current user object.

Fixed #497.
Fix: cart is not initialized when some shipping modules are triggered
Only flush theme transients if the old theme engine is being active.
Avoid serializing and saving the cart in wp-admin area
Merge pull request #498 from garyc40/pr-497

Make sure customer data is properly initialized after WordPress core has finished initializing current user object