Allow magic finders to use underscore columns

We have an existing application that uses underscores in the database column names. The magic finders don't allow this kind of column naming.

This patch is the first of a few that are required for full support of underscore naming conventions.

`::findByProductIdAndOrderId()` will now resolve to `order_id` and `product_id` if it is defined in the schema. Otherwise, it resolves to `orderid` and `productid`.