Admin: Purchase search fails when multiple URL parameters are present

[vatsalkaushik]

When searching for purchases by email using the Search Purchases field in admin, no results are returned even though there are valid purchases for that email.

This appears to be caused by additional URL parameters interfering with the search. For example:

• Does not work: https://gumroad.com/admin/search/purchases?query=email%40example.com&user_query=&purchase_query=email%40example.com&affiliate_query=&product_title_query=&purchase_status=&card_type=&transaction_date=&last_4=&expiry_date=&price=

• Works: https://gumroad.com/admin/search/purchases?query=email%40example.com