Kestrel does not send "100 Continue" when MinRequestBodyDataRate is null

[halter73]

Kestrel does not send "100 Continue" when MinRequestBodyDataRate is null. TryProduceContinue should still be called even if _timingEnabled is false in the following logic:

aspnetcore/src/Servers/Kestrel/Core/src/Internal/Http/MessageBody.cs

Lines 185 to 187 in 8df6490

	if (!readAwaitable.IsCompleted && _timingEnabled)
	{
	TryProduceContinue();

The fix will need to be backported to release/3.1.

[halter73]

Fixed by #31284