Exceptions with long messages don't appear in Crashlytics dashboard

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

* Unity editor version: 2018.4.17f1
* Firebase Unity SDK version: 6.15.2
* Source you installed the SDK: Package Manager
* Problematic Firebase Component: Crashlytics
* Other Firebase Components in use: Analytics
* Additional SDKs you are using: irrelevant
* Platform you are using the Unity editor on: Windows
* Platform you are targeting: iOS & Android
* Scripting Runtime: IL2CPP

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

When logging an exception with a really long message, the exception doesn't appear in the dashboard:
Crashlytics.LogException(new Exception(message, e));

#### Steps to reproduce:

Have you been able to reproduce this issue with just the Firebase Unity quickstarts (this GitHub project)?
What's the issue repro rate? 100%

What happened? How can we make the problem occur?
Log an exception with a 100 characters message -> error appears in Firebase Crashlytics dashboard
Log an exception with a 10.000 characters message -> error doesn't appear in Firebase Crashlytics dashboard