Use task.IsCompletedSuccessfully rather than ReferenceEquals

[benaadams]

As ASP.NET Core now targets .NET Core it can use task.IsCompletedSuccessfully; this is preferable to comparing to Task.CompletedTask as an async method uses a different completed task than Task.CompletedTask so it will catch more instances.

Also will make @stephentoub happier 😄

[stephentoub]

so it will catch more instances

Including asynchronously completing operations that then complete fast enough that they're complete by the time the check happens.

[davidfowl]

Much better,

[rynowak]

<3

[benaadams]

Rebased