HTTP2: Header enumerator allocations

[JamesNK]

Header enumerator allocations show up:

Boxing + allocating a new enumerable:

aspnetcore/src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs

Lines 666 to 675 in 7a4460c

	private static IEnumerable<KeyValuePair<string, string>> EnumerateHeaders(IHeaderDictionary headers)
	{
	foreach (var header in headers)
	{
	foreach (var value in header.Value)
	{
	yield return new KeyValuePair<string, string>(header.Key, value);
	}
	}
	}

IEnumerable is used by QPackEncoder.BeginEncode which is shared code

aspnetcore/src/Shared/runtime/Http3/QPack/QPackEncoder.cs

Lines 344 to 357 in 7a4460c

	public bool BeginEncode(IEnumerable<KeyValuePair<string, string>> headers, Span<byte> buffer, out int length)
	{
	_enumerator = headers.GetEnumerator();
	bool hasValue = _enumerator.MoveNext();
	Debug.Assert(hasValue == true);
	buffer[0] = 0;
	buffer[1] = 0;
	return Encode(buffer.Slice(2), out length);
	}
	public bool BeginEncode(int statusCode, IEnumerable<KeyValuePair<string, string>> headers, Span<byte> buffer, out int length)