Avoid a TypeError when ChoiceFilter choices are a callable

[glennmatthews]

Fixes #688.

[tfranzel]

many thanks... just one minor comment and a suggestion.

I also added this to test_django_filters_requests to make sure this behaves as expected

+    response = APIClient().get('/api/products/?choice_func=C')                                                                                                                                                      
+    assert response.status_code == 200                                                                                                                                                                              
+    assert len(response.json()) == 0  

[codecov]

Codecov Report

Merging #689 (acf26a4) into master (fdf07fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #689   +/-   ##
=======================================
  Coverage   98.86%   98.87%           
=======================================
  Files          64       64           
  Lines        7167     7175    +8     
=======================================
+ Hits         7086     7094    +8     
  Misses         81       81           

Impacted Files	Coverage Δ
drf_spectacular/contrib/django_filters.py	92.45% <100.00%> (+0.14%)	⬆️
tests/contrib/test_django_filters.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fdf07fe...acf26a4. Read the comment docs.