Prevent the content-length header field from being duplicated.

[beb63]

This change fixes a bug where content-length (and potentially also content-range) header fields were added to the list of response header fields without any regard to already included such header field(s).

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.14%. Comparing base (05e24b1) to head (457c467).
⚠️ Report is 48 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   76.30%   76.14%   -0.17%     
==========================================
  Files          12       12              
  Lines         764      742      -22     
==========================================
- Hits          583      565      -18     
+ Misses        181      177       -4     

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