fix: Missing media_type in the Case.partial_deepcopy implementation.

[Stranger6667]

It led to missing payload in failure reproduction code samples

[codecov-commenter]

Codecov Report

Merging #1428 (33c30d1) into master (1e276cd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1428   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files          86       86           
  Lines        6524     6524           
  Branches     1139     1139           
=======================================
  Hits         6466     6466           
  Misses         30       30           
  Partials       28       28           

Flag	Coverage Δ
unittests	99.09% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/schemathesis/models.py	99.05% <100.00%> (ø)
src/schemathesis/cli/callbacks.py	98.33% <0.00%> (-1.67%)	⬇️
src/schemathesis/runner/impl/threadpool.py	100.00% <0.00%> (+1.76%)	⬆️

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