Fix descending choices building for OrderingFilter

[rpkilby]

Alternate to #1176, fixes #1175.

Note that this might be made moot by #1118, although a similar test case should be incorporated.

[codecov-io]

Codecov Report

Merging #1183 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1183      +/-   ##
==========================================
+ Coverage   99.43%   99.44%   +<.01%     
==========================================
  Files          15       15              
  Lines        1243     1259      +16     
==========================================
+ Hits         1236     1252      +16     
  Misses          7        7

Impacted Files	Coverage Δ
django_filters/filters.py	100% <100%> (ø)	⬆️
django_filters/widgets.py	100% <0%> (ø)	⬆️
django_filters/filterset.py	100% <0%> (ø)	⬆️
django_filters/conf.py	100% <0%> (ø)	⬆️

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[carltongibson]

My initial glance[*] at #1118 was that it looked super, so... — Let's leave this hanging for a bit.

[*]: didn't have time to give it it's due.