Fix distinct call for range filters

[rpkilby]

Per @joinworldtv with #417 (comment)

Instead of reimplementing the .filter() method entirely, the override simply alters the .lookup_expr and value, then calls the base method.

[codecov-io]

Codecov Report

Merging #855 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
+ Coverage   97.95%   98.12%   +0.17%     
==========================================
  Files          15       15              
  Lines        1122     1121       -1     
==========================================
+ Hits         1099     1100       +1     
+ Misses         23       21       -2

Impacted Files	Coverage Δ
django_filters/filters.py	99.05% <100%> (+0.62%)	⬆️

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[carltongibson]

So surely there's a missing test case here? 🙂

[rpkilby]

So surely there's a missing test case here? 🙂

I was thinking about that. The updated .filter() method is simply altering the lookup expression and value, before calling the base implementation. The current tests sufficiently cover that behavior, so it didn't seem necessary to add specific tests for compatibility with the other features (like exclude/distinct).

That said a sanity check wouldn't hurt.

[rpkilby]

Okay, updated with some sanity tests for calling distinct.