Backport PR #965: Correct content-type headers

[epignot]

(cherry-picked from commit 15310fd)

[codecov-commenter]

Codecov Report

Merging #966 (a42cac5) into 1.x (df59cfe) will decrease coverage by 0.04%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##              1.x     #966      +/-   ##
==========================================
- Coverage   71.48%   71.43%   -0.05%     
==========================================
  Files          62       62              
  Lines        7733     7738       +5     
  Branches     1283     1283              
==========================================
  Hits         5528     5528              
  Misses       1816     1816              
- Partials      389      394       +5     

Impacted Files	Coverage Δ
jupyter_server/gateway/gateway_client.py	71.18% <0.00%> (-1.55%)	⬇️
jupyter_server/gateway/managers.py	81.26% <100.00%> (ø)
jupyter_server/services/contents/filemanager.py	71.40% <0.00%> (-0.80%)	⬇️
jupyter_server/serverapp.py	65.24% <0.00%> (-0.19%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	80.58% <0.00%> (+1.94%)	⬆️

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[blink1073]

Thanks again!