shopping API uses X-EBAY-API-SITE-ID not X-EBAY-API-SITEID

This is a fix for the shopping API to use sites other than the default.

There is a similar X-EBAY-API-SITEID on trading and merchandising classes.  I don't think merchandising used either spelling of the SITE-ID header as far as I can tell and I ran out of patience with the documentation before I checked on trading.  I don't know if others like finding would need something similar.