Complete IPipeWriter in OutputProducer.Abort() (#1894)

- This prevents MemoryPoolBlocks from leaking when writes fail

src/Microsoft.AspNetCore.Server.Kestrel.Core/Internal/Http/OutputProducer.cs

@@ -95,6 +95,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
                 _log.ConnectionDisconnect(_connectionId);
                 _completed = true;
                 _pipe.Reader.CancelPendingRead();
+                _pipe.Writer.Complete();
             }
         }