handle already converted LogLevel and JSON

[graingert]

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Description

[graingert]

see https://github.com/celery/celery/pull/6861/files#diff-5a6573970352ba5472491b51b8914b8dcece9444713edd8165f53adc34b781abR49

[codecov]

Codecov Report

Merging #6915 (3f28cdd) into master (8bff307) will decrease coverage by 0.08%.
The diff coverage is 34.61%.

@@            Coverage Diff             @@
##           master    #6915      +/-   ##
==========================================
- Coverage   89.29%   89.20%   -0.09%     
==========================================
  Files         138      138              
  Lines       16632    16653      +21     
  Branches     2101     2106       +5     
==========================================
+ Hits        14851    14856       +5     
- Misses       1561     1577      +16     
  Partials      220      220              

Flag	Coverage Δ
unittests	89.20% <34.61%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
celery/bin/base.py	44.38% <32.00%> (-2.61%)	⬇️
celery/bin/call.py	88.88% <100.00%> (ø)

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[thedrow]

Thanks!