## Summary
- Fixes#26422
- Preserve HTTP header case when sending requests (e.g., `Content-Type`
instead of `content-type`)
- HTTP headers are technically case-insensitive per RFC 7230, but many
APIs expect specific casing
## Test plan
- [x] Added tests that verify headers are sent with proper case on the
wire
- [x] Tests use raw TCP sockets to capture actual HTTP wire format
- [x] Tests fail with system Bun (lowercase headers), pass with fixed
build
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