Reproduction Steps:
1. Import attached project.
2. Open "start" scene.
3. Enter Play mode.
4. Select "Case unsensitive scene name" button in scene.

Expected result: "TestScene" is loaded, similar to how it is loaded when "SceneManager.LoadSceneAsync(SceneName, LoadSceneMode.Single);" is called.

Actual result: Unity throws out error when scene is loaded with "SceneName.ToLower()" or "SceneName.ToUpper()":
Scene 'testscene' couldn't be loaded because it has not been added to the build settings or the AssetBundle has not been loaded.
To add a scene to the build settings use the menu File->Build Settings...

Reproducible with: 5.5.3p2, 5.6.0p3, 2017.1.0b2

Note: No backwards compatibility for previous versions.