Ensure content-type properly reflects gateway kernelspec resources

[kevin-bates]

The APIHandler.finish() method unconditionally overrides the header's Content-type to application/json despite the GatewayResourceHandler setting it to a mime-type corresponding to the resource's extension.

This pull request enables the caller to indicate the ContentType that should be set into the header in APIHandler.finish() where application/json is the default if no content type is specified.

Resolves: #216

[codecov]

Codecov Report

Base: 80.46% // Head: 80.45% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (c7739fb) compared to base (ce8e7fc).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1219      +/-   ##
==========================================
- Coverage   80.46%   80.45%   -0.01%     
==========================================
  Files          68       68              
  Lines        8134     8135       +1     
  Branches     1586     1586              
==========================================
  Hits         6545     6545              
- Misses       1174     1176       +2     
+ Partials      415      414       -1     

Impacted Files	Coverage Δ
jupyter_server/base/handlers.py	78.86% <100.00%> (+0.04%)	⬆️
jupyter_server/gateway/handlers.py	62.27% <100.00%> (ø)
...ter_server/services/kernels/connection/channels.py	61.72% <0.00%> (-0.45%)	⬇️
jupyter_server/serverapp.py	80.05% <0.00%> (+0.08%)	⬆️

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[blink1073]

Thank you!