[Bug] Remote Config FetchAsync throws exception with TimeSpan

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2020.3.18f1
* Firebase Unity SDK version: 8.1.0
* Source you installed the SDK: Unity Package Manager
* Problematic Firebase Component: Remote Config
* Other Firebase Components in use: Firestore, Crashlytics, Auth
* Additional SDKs you are using:
* Platform you are using the Unity editor on: Windows
* Platform you are targeting: Android (Unity Editor is also affected)
* Scripting Runtime: IL2CPP

### [REQUIRED] Please describe the issue here:

When setting the cache expiration of `FetchAsync()` to `TimeSpan.Zero` (or any other TimeSpan), an exception is thrown with `ErrorCode 1`. The `Message` parameter is empty.

#### Steps to reproduce:

#### Relevant Code:

```
await FirebaseRemoteConfig.DefaultInstance.FetchAsync(TimeSpan.Zero); //This (or any other TimeSpan) will result in an exception
```