Support many related fields on plain Serializers

[nattyg93]

Fixes #704

Description of the Change

Fix issue preventing resolving the resource type of ResourceRelatedField(many=True) fields on plain serializers

Checklist

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

[codecov]

Codecov Report

Merging #868 (9d654fe) into master (3263061) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #868   +/-   ##
=======================================
  Coverage   97.74%   97.74%           
=======================================
  Files          60       60           
  Lines        3319     3319           
=======================================
  Hits         3244     3244           
  Misses         75       75           

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[sliverc]

Thanks for working on this! See my inline comment.

[sliverc]

LGTM. Thanks for your work.