added custom fields failing test case

Following your last suggestion on #1479 I prepared a test case, with this I hope to understand if I'm doing anything wrong or if there really is an issue with DRF.

The suggestion was:

federico:
> the field was not visible on the API browser after a certain version of DRF, which I think it's 2.3.9.

tom:
> If so then that's the issue that actually needs resolving here. I'd suggest working towards getting that resolved instead - figure out a failing test case, figure out exactly which commit broke the behaviour and then look towards fixing the issue.

In this test case I try to do 2 things:
 1. use restore_object to pop out the **"password_confirmation"** field
 2. ensure the **"password_confirmation"** field shows up in the API browser

It results that using only `restore_object` is not enough, so I try to override the `to_native` method by popping up the field there too, but that results in the field to not show up in the form of the API browser anymore.

@un33k @stonaz