[Rust-Axum] Fix: handling non-lowercase Header in Request/Response

[linxGnu]

This PR prevent panic in http while handling non-lowercase HTTP Header name.

Passed Integration Tests:

mvn clean && ./bin/generate-samples.sh bin/configs/manual/rust-axum-* && mvn integration-test -f samples/server/petstore/rust-axum/pom.xml

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package || exit
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  ./bin/utils/export_docs_generators.sh || exit
  

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[linxGnu]

Kindly ping @wing328 to review