Allow repeated filter query parameters

[ro70]

Fixes #718

Description of the Change

Allows repeated filter query parameters like ?filter[authors.id]=1&filter[authors.id]=2.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #782 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
+ Coverage   97.36%   97.39%   +0.02%     
==========================================
  Files          56       56              
  Lines        2962     2989      +27     
==========================================
+ Hits         2884     2911      +27     
  Misses         78       78              

Impacted Files	Coverage Δ
example/tests/test_filters.py	100.00% <100.00%> (ø)
rest_framework_json_api/django_filters/backends.py	100.00% <100.00%> (ø)
rest_framework_json_api/filters.py	100.00% <100.00%> (ø)

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[sliverc]

This looks good in general. Thanks for working on it. As of my comment let's wait for confirmation form @n2ygk . Besides a changelog entry needs to be added.

[auvipy]

rebase needed

[sliverc]

Thanks!