fix: Header matching case-sensitivity fixed

[dadrus]

Related issue(s)

closes #472

Checklist

• I agree to follow this project's Code of Conduct.
• I have read, and I am following this repository's Contributing Guidelines.
• I have read the Security Policy.
• I have referenced an issue describing the bug/feature request.
• I have added tests that prove the correctness of my implementation.

Description

Closes a bug related to header matching during error handler execution

[codecov]

Codecov Report

Merging #483 (4ee703e) into main (ffb3668) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #483      +/-   ##
==========================================
+ Coverage   88.32%   88.37%   +0.05%     
==========================================
  Files         194      194              
  Lines        8002     8003       +1     
==========================================
+ Hits         7068     7073       +5     
+ Misses        764      760       -4     
  Partials      170      170              

Impacted Files	Coverage Δ
...mechanisms/errorhandlers/matcher/header_matcher.go	100.00% <100.00%> (ø)
internal/rules/repository.go	97.29% <0.00%> (+3.60%)	⬆️

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