Issue 997: Calling async ServerRpc that returns value crashes the server on 2021.2

Repository: miragenet/mirage

Issue body:
**Describe the bug**
Simply calling an async UniTask ServerRpc from a client instantly crashes the server, whether it be in the editor or on a build, with Unity 2021.2. Also tested on 2021.1.13, but there was no issue there.

**Repro project**
Tiny project with a simple Player script that calls an async rpc to get randomized X position when spacebar is pressed.
[Mirage Async RPC Test.zip](https://github.com/MirageNet/Mirage/files/7628640/Mirage.Async.RPC.Test.zip)

**To Reproduce**
Steps to reproduce the behavior:
1. Open the attached project.
2. Open the 'Scene' scene.
3. Build a player or duplicate the project so multiplayer can be tested.
4. Play game, host. Join with other client.
5. Press Spacebar key on the joined client to call the async rpc, instantly crashing the server process.

**Desktop (please complete the following information):**
* OS: Windows 10
* Build target: Editor/Windows
* Unity version: 2021.2.4
* Mirage version: v113.0.0

**Additional context**
The calling code looks like this:
```cs
public class Player : NetworkBehaviour
{
    private void Update()
    {
        if (IsLocalPlayer && Input.GetKeyUp(KeyCode.Space))
            StrafeByRandomValueFromServer().Forget();
    }

    private async UniTaskVoid StrafeByRandomValueFromServer()
    {
        float value = await RequestRandomValue();
        transform.position = new Vector3(value, 0f, 0f);
    }

    [ServerRpc]
    public UniTask<float> RequestRandomValue()
    {
        return UniTask.FromResult(Random.Range(-2, 2f));
    }
}
```

Crash log:
[Player.log](https://github.com/MirageNet/Mirage/files/7628746/Player.log)
