fixed 204/304 HTTPException handling

[bentheiii]

1. responses with 204/304 status codes shouldn't ever have content
2. If headers are provided as part of the HTTPException, their "content-length" and "content-type" should be removed, since they'll need to be overridden for the new response.

closes #4949

[codecov]

Codecov Report

Merging #4952 (5280307) into master (ab8988f) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##            master    #4952      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files          533      533              
  Lines        13725    13734       +9     
===========================================
+ Hits         13725    13732       +7     
- Misses           0        2       +2     

Impacted Files	Coverage Δ
fastapi/exception_handlers.py	90.90% <85.71%> (-9.10%)	⬇️

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[github-actions]

📝 Docs preview for commit b2d5b28 at: https://628e3e766bc9ba22f5eba971--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 78a589a at: https://628e3f8fe8c1f9239209a94e--fastapi.netlify.app

[fboerman]

this seems to be a fix for #4939

[fboerman]

I can confirm this fixes my problem when using:

raise HTTPException(status_code=HTTP_304_NOT_MODIFIED)

[tiangolo]

Thanks for the contribution! It seems this was handled in #5145, available since FastAPI 0.79.0. So I'll close this one. But if you feel there's anything else to be done, feel free to add a new PR. Thanks! ☕