Fix multiple bugs: null checks, header case-sensitivity, async patterns, test assertions

[Copilot]

Summary

This PR fixes several bugs identified through code review of the codebase.

Changes

1. Fix ArgumentNullException.ThrowIfNull(nameof(x)) — always a no-op

Files: HttpResponseHeaders.cs, Http2FrameWriter.cs, MvcMiddleware.cs, Controller.cs

ThrowIfNull(nameof(value)) passes the string literal "value" which is never null — making the check useless. Fixed to pass the actual variable.

2. Fix test assertion argument order (expected/actual swapped)

File: ReadRequestTests.cs

Assert.AreEqual(actual, expected) produces misleading failure messages. Fixed to Assert.AreEqual(expected, actual).

3. Fix HttpHeaders.Remove crashes when header doesn't exist

File: HttpHeaders.cs

FindIndex returns -1 when not found, but the code directly indexed headers[-1], causing IndexOutOfRangeException. Added bounds check.

4. Fix case-sensitive header name comparisons

File: HttpHeaders.cs

HTTP headers are case-insensitive per RFC 7230. AddOrUpdate, AddOrSkip, and Remove used == for comparison. Fixed to use StringComparison.OrdinalIgnoreCase (consistent with the existing TryGetValue method).

5. Fix wrong exception type in MultipartFormDataFormReader

File: MultipartFormDataFormReader.cs

Was throwing NullReferenceException (a runtime-only exception) for invalid state. Changed to InvalidOperationException which correctly signals programmer error.

6. Fix Task.WaitAll blocking in async context

File: MiniWebClientConnection.cs

Replaced synchronous Task.WaitAll() with await Task.WhenAll() to prevent thread pool starvation and potential deadlocks in async methods.