Wrong error logging when a client cancels the request

[maxcherednik]

Describe the bug

When a client cancels the web request(closing a browser for instance) the request is cancelled with a help of the CancelationToken. Everything works fine but this log line is written into the log file:

2019-03-19T08:37:13.093+00:00 [17] ERR Microsoft.AspNetCore.Server.HttpSys.HttpSysListener [/main] - ResponseStream::Dispose; Fewer bytes were written than were specified in the Content-Length.

The reason is this line in the ResponseBody class: https://github.com/aspnet/AspNetCore/blob/5f4be179e3bf6d88a53667935fc5c4bd2d71988c/src/Servers/HttpSys/src/RequestProcessing/ResponseBody.cs#L131

This line is executed when a server calls Dispose on the ResponseBody instance and in general it is good have this check. But now it should have a distinction between the normal flow and the flow when a client is not connected anymore.

To Reproduce

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Expected behavior

There should not be an error log like this:

2019-03-19T08:37:13.093+00:00 [17] ERR Microsoft.AspNetCore.Server.HttpSys.HttpSysListener [/main] - ResponseStream::Dispose; Fewer bytes were written than were specified in the Content-Length.

[Tratcher]

Yes, that error message is redundant and could be suppressed for this scenario. A change like this is low priority for us but you're welcome to send a PR.