De-duplicate base headers if specified by user

[sjberman]

Problem: If a user set a header in a header filter that had a default value set by NGF, both values would end up being set.

Solution: If provided by a user, don't set the default header value.

Testing: Unit test and manual verification of header de-duplication.

Closes #3192

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Fix an issue where default headers were still being set when overwritten by a user.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.16%. Comparing base (60b6b19) to head (87a638e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3249   +/-   ##
=======================================
  Coverage   86.15%   86.16%           
=======================================
  Files         113      113           
  Lines       11658    11664    +6     
  Branches       62       62           
=======================================
+ Hits        10044    10050    +6     
  Misses       1553     1553           
  Partials       61       61           

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