Fixing potentially ambiguous properties in get_orders()

[dparker1005]

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Changes proposed in this Pull Request:

When both id and discount_code_id are used, the id column in the database is ambiguous.

This change defines the table for all search properties in get_orders().

Resolves XXX.

How to test the changes in this Pull Request:

$order_args = array(
'id' => '82',
'discount_code_id' => '2',
);
var_dump( MemberOrder::get_orders( $order_args ) );

Returns empty array before the fix, even if there is a matching order. After the fix, returns matching orders.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.