Populate $current_user when users create an account during checkout

Currently - if a user chooses to have an account created for them during checkout, WC sets up their auth cookie so that they are logged in, but doesn't set up the global $current_user object. The result is that any code that runs in the same page request, and uses standard WP functions such as wp_get_current_user() fails to recognise them as a registered user. 

A possibly similar fix is needed to process_registration in woocommerce-functions.php. We probably could have got away without it - since the code did a redirect virtually straight after, but now it seems there's an hook called it seems sensible to set up the $current_user object there as well?

I can prepare a revised pull request if required?