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Fix improper memory reuse in NewFastHTTPHandler ( 1860)

Fix improper memory reuse in NewFastHTTPHandler ( 1860): a task in repo2rlenv-commit-runtime-v2 (Harbor dataset). Title: NewFastHTTPHandler causes data corruption or incorrect header/cookie values due to memory reuse

The task

**Title:** NewFastHTTPHandler causes data corruption or incorrect header/cookie values due to memory reuse

Part of FineEnvs/repo2rlenv-commit-runtime.