Use task_serializer for doing roundtrip serialization for eager tasks

[bjackson]

Description

Fixes #5133
When doing the roundtrip serialization for eager tasks, the task serializer will always be JSON unless the serializer argument is present in the call to Task.apply_async. If the serializer argument is present but is 'pickle', an exception will be raised as pickle-serialized objects cannot be deserialized without specifying to serialization.loads what content types should be accepted. The Producer's serializer seems to be set to None, causing the default to JSON serialization.

This PR will continue using (in order) the serializer argument to Task.apply_async, if present, or the Producer's serializer if not None. If the Producer's serializer is None, it will use the Celery app's task_serializer configuration entry as the serializer.

[auvipy]

it would be great if you could add a test for this!

[bjackson]

I'll add one.

[codecov]

Codecov Report

Merging #5342 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5342   +/-   ##
=======================================
  Coverage   83.56%   83.56%           
=======================================
  Files         143      143           
  Lines       16335    16335           
  Branches     2030     2030           
=======================================
  Hits        13651    13651           
  Misses       2481     2481           
  Partials      203      203

Impacted Files	Coverage Δ
celery/app/task.py	96.56% <ø> (ø)	⬆️

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