pytest_plugin: allow user specified headers in jp_ws_fetch

[oliver-sanders]

Fixes a small error in the jp_ws_fetch fixture where the headers provided to client_fetch were ignored in requests.

[codecov-commenter]

Codecov Report

Merging #580 (7c41332) into master (49d5772) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 7c41332 differs from pull request most recent head a88e36f. Consider uploading reports for the commit a88e36f to get more accurate results

@@            Coverage Diff             @@
##           master     #580      +/-   ##
==========================================
+ Coverage   77.46%   77.52%   +0.05%     
==========================================
  Files         110      110              
  Lines       10184    10184              
  Branches     1100     1100              
==========================================
+ Hits         7889     7895       +6     
+ Misses       1907     1899       -8     
- Partials      388      390       +2     

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	90.81% <100.00%> (ø)
jupyter_server/services/kernels/kernelmanager.py	81.84% <0.00%> (+2.05%)	⬆️

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[blink1073]

Thank you!