Fix reverse m2m relation update

[bierik]

Fixes #165

[codecov-commenter]

Codecov Report

Merging #167 (ae3c6f6) into master (a3d53dc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #167   +/-   ##
=======================================
  Coverage   98.18%   98.19%           
=======================================
  Files           3        3           
  Lines         220      221    +1     
=======================================
+ Hits          216      217    +1     
  Misses          4        4           

Flag	Coverage Δ
unittests	98.19% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
drf_writable_nested/mixins.py	98.08% <100.00%> (+<0.01%)	⬆️

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