Preserve header case in IOClient

[natebosch]

Closes #609

Some non-compliant servers misbehave with the default handling of
headers in the dart:io HTTP implementation. Well behaved servers
should not be impacted, but other use cases which require a particular
formatting can be handled by always preserving header case.

[github-actions]

PR Health

License Headers ✔️

// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/cupertino_http/example/example.dart
pkgs/http/example/main.dart
pkgs/http_multi_server/test/cert.dart

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
http	None	1.6.0	1.6.1-wip	1.6.1-wip	✔️

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Coverage ✔️

File	Coverage
pkgs/http/lib/src/io_client.dart	💚 89 %

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

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Unused Dependencies ⚠️

Package	Status
http	❗ Show Issues These packages may be unused, or you may be using assets from these packages: * dart_flutter_team_lints * shelf

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Changelog Entry ✔️

Package	Changed Files

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[brianquinlan]

Is it worth adding a test for this?

[natebosch]

Is it worth adding a test for this?

Can't hurt. Done.