Fix an unobserved task exception (#188)

* Fix an unobserved task exception

Addresses an unobserved exception that occurs when an `ObjectDisposedException` is raised after a `CancellationToken` is cancelled by only cancelling if the token has not been previously cancelled. The default `catch` will fire for an `OperationCanceledException` and then attempt to cancel the same `CancellationToken` that was just cancelled.

* `try/catch` the call to `Cancel`, add a comment

Author: Dean Ward

src/protobuf-net.Grpc/Internal/Reshape.cs

@@ -520,7 +520,14 @@ static async Task SendAll<T>(IClientStreamWriter<T> output, IAsyncEnumerable<T>
             }
             catch
             {
-                allDone.Cancel();
+                if (!allDone.IsCancellationRequested)
+                {
+                    try
+                    {
+                        allDone.Cancel();
+                    }
+                    catch { } // calls to "Cancel" can race, ignore the exception if we lose the race
+                }
                 throw;
             }
         }