cacheable-request - fix: 304 Not Modified has completely empty headers

[jaredwray]

Please check if the PR fulfills these requirements

• Followed the Contributing guidelines.
• Tests for the changes have been added (for bug fixes/features) with 100% code coverage.
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
cacheable-request - fix: 304 Not Modified has completely empty headers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d1f53a1) to head (b800bfc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1409   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         4140      4163   +23     
  Branches      1101      1153   +52     
=========================================
+ Hits          4140      4163   +23     

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