Fix descending choices building for OrderingFilter

[jock-tanner]

Fixes #1175

[codecov-io]

Codecov Report

Merging #1176 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1176      +/-   ##
==========================================
+ Coverage   99.43%   99.43%   +<.01%     
==========================================
  Files          15       15              
  Lines        1243     1244       +1     
==========================================
+ Hits         1236     1237       +1     
  Misses          7        7

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

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[rpkilby]

Hi @jock-tanner. I tried pushing a few changes to your branch, but was unable to. Closing in favor of #1183. Thanks!

[jock-tanner]

@rpkilby OK now I see my timing with this PR was no good considering the upcoming OrderingFilter refactoring in #1118. Thank you for taking your time nevertheless.