Update Lookup.php

Whilst using this library and amazon's advertising product API I've come across the following errors: 

"Message":"Your request contained a restricted parameter combination. When IdType equals ASIN, SearchIndex cannot be present."
and 
"Your request is missing a required parameter combination. When IdType equals ['DPCI', 'SKU',\n\t\t\t\t'UPC', 'EAN','ISBN'], SearchIndex must be present."

as such I've modified Apai-io locally to the same as this commit