Don't call CompleteReader, the dispatcher can handle that

src/Microsoft.AspNetCore.Sockets/ServerSentEvents.cs

@@ -36,8 +36,6 @@ namespace Microsoft.AspNetCore.Sockets
 
                 _channel.Output.Advance(buffer.End);
             }
-
-            _channel.Output.CompleteReader();
         }
 
         private async Task Send(HttpContext context, ReadableBuffer data)

src/Microsoft.AspNetCore.Sockets/WebSockets.cs

@@ -109,8 +109,6 @@ namespace Microsoft.AspNetCore.Sockets
                 }
             }
 
-            _channel.Output.CompleteReader();
-
             // REVIEW: Should this ever happen?
             if (!IsClosedOrClosedSent(ws))
             {