Added test for #1408 — Already Passes

This was an attempt to reproduce #1408. 

It checks that a blank option is added to the `widget.choices` of a `RelatedField` if `required=False`.

The test already passes without further changes. 

Since #1408 was reported against DRF v2.2.6 which is now quite out of date I will assume it was fixed in the meantime. 

Closes #1408