Address comments of https://github.com/django-parler/django-parler/pull/216

[shun-liang]

This PR aims to address the comments of django-parler#216

[codecov-io]

Codecov Report

Merging #1 into master will increase coverage by 1.48%.
The diff coverage is 98.3%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   85.73%   87.22%   +1.48%     
==========================================
  Files          31       31              
  Lines        1900     1933      +33     
==========================================
+ Hits         1629     1686      +57     
+ Misses        271      247      -24

Impacted Files	Coverage Δ
parler/tests/test_querysets.py	100% <100%> (ø)	⬆️
parler/managers.py	91.21% <100%> (+4.05%)	⬆️
parler/tests/testapp/models.py	94.32% <100%> (+0.57%)	⬆️
parler/utils/fields.py	96.29% <95.45%> (+69.37%)	⬆️

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