Fix Content Length Writer

[mpass99]

• The Content-Length-Header did not get removed in the error handling
• The Content-Length Writer fired the HTTP Headers unnecessarily early impeding sending of an error message.

Closes #595

[MrSerth]

Thanks, looks good and fixes the actual problem (I reproduced it first with CodeOcean locally and then upgraded Poseidon to see that it got resolved) 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.73%. Comparing base (06deab0) to head (6c298b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #652      +/-   ##
==========================================
+ Coverage   76.63%   76.73%   +0.10%     
==========================================
  Files          41       41              
  Lines        3509     3512       +3     
==========================================
+ Hits         2689     2695       +6     
+ Misses        608      605       -3     
  Partials      212      212              

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