Updated header for wpsc_update_customer_cart to reflect 3.8.14
Changed order of paramters in wpsc_update_customer_cart.  'id' is now last and optional, consistant with other customer meta functions.
In merge cart, check to see if headers have already been sent before clearing the WPEC cookie.
Discussed in this support thread: http://wordpress.org/support/topic/wpec-creates-new-users-each-time-i-load-a-page?replies=12#post-5197801
Merge branch 'customer-meta-hardening'