Accept str for ex Parameter in 'set' Command

[shacharPash]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
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Description of change

Please provide a description of the change here.

Fixes #2178 issue

[codecov-commenter]

Codecov Report

Base: 92.22% // Head: 92.23% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (0cb6d46) compared to base (f06f3db).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2529   +/-   ##
=======================================
  Coverage   92.22%   92.23%           
=======================================
  Files         113      113           
  Lines       29375    29383    +8     
=======================================
+ Hits        27092    27100    +8     
  Misses       2283     2283           

Impacted Files	Coverage Δ
redis/commands/core.py	82.18% <100.00%> (+0.01%)	⬆️
tests/test_commands.py	90.00% <100.00%> (+0.01%)	⬆️
tests/test_cluster.py	96.90% <0.00%> (-0.12%)	⬇️
tests/test_asyncio/test_cluster.py	97.60% <0.00%> (+0.12%)	⬆️

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