Steps to repro:

1. Open attached project and 'test' scene;
2. Observe Scene and Game views.

Actual result:
Shadows are not casted on terrain when using Spotlight. See attached actual.png screenshot.

Expected result:
Shadows are casted on terrain from terrain itself and all objects that could cast shadows. See attached expected.png screenshot.

Notes:
- Reproducible in 5.6.0b3, 5.5.0p1, 5.4.3f1;
- Reproducible on OSX 10.12.1 and Windows 10;
- Also reproducible for Point light;
- Not reproducible when using Directional Light;
- Reproducible for Built in Legacy Diffuse, Built in Legacy Specular terrain materials;
- Reproducible for Nature/Terrain/Diffuse and Nature/Terrain/Specular shaders;
- Not reproducible for Nature/Terrain/Standard shader;
- Reproducible in both Editor and Standalone build;
- Reproducible for both Stationary and Dynamic light modes.