test: cover normalized Request headers

[AbhirupKarmakar]

Summary

Add a test for Request receiving an already-normalized Headers instance. It verifies the instance is retained and its original header casing remains available through raw_items(). This covers existing behavior; production code is unchanged.

Verification

• python -m pytest --cov=h11 --cov-config=.coveragerc h11 (79 passed)
• Temporarily disabled the normalized-headers fast path and confirmed the test failed because the header instance was replaced, then restored the source.
• Baseline coverage had not exercised the Request constructor's Headers input branch.