Setting Default Server map resets Default Game Mode to GameModeBase

REGRESSION: Yes, doesn't occur in 4.15

When the user modifies the server default map it resets the game mode in DefaultEngine.ini

This occurs on both Mac and Windows.

1. Open QAGame.
2. Select Edit > Project Settings.
3. Expand the Default Maps settings.
4. Set Default Server Map to any map (such as QA-Promotion).
5. Run UE4Editor.exe QAGame -server -log
6. Run UE4Editor.exe QAGame <Your IP Address> -Game

RESULT

User will load into the level with no Game Mode (will just be a floating camera). When opening Project Settings up after this, you can see that the Default Game Mode was set to GameModeBase.