Bug in descending choices building for OrderingFilter

[jock-tanner]

I got a problem assigning custom labels to reverse ordering options with field_labels parameter. I think this method is incorrect in its descending = ... part. It looks up for -param instead of -field, as stated in the docs: «Keep in mind that the key is the field name, and not the exposed parameter name».

I have a fix for this issue, including an upgrade of the current test case with this issue in mind. I will provide it ASAP. If anything is wrong with my fix, please let me know. I'd like it to work for everybody, not just me.