Correct 'Content-Type' headers

[faucct]

jupyter/notebook#6026

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[codecov-io]

Codecov Report

Merging #471 (588b5be) into master (723786e) will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##           master     #471   +/-   ##
=======================================
  Coverage   77.82%   77.82%           
=======================================
  Files         106      106           
  Lines        9127     9132    +5     
  Branches      976      979    +3     
=======================================
+ Hits         7103     7107    +4     
  Misses       1676     1676           
- Partials      348      349    +1     

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	76.87% <88.88%> (+0.04%)	⬆️

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[blink1073]

Thanks again @faucct!

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Congrats on your first merged pull request in this project! 🎉

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