Description: Custom Shader Editor doesn't refresh when changed in the shader code.

To reproduce:

1. Load shadertest project.
2. Open the def/myshader.shader file, scroll to the end, and make sure that 'CustomEditor "StandardShaderGUI"' is commented out.
3. Open 'def' scene.
4. Click on 'Cube' in the hierachy.
5. In the inspector for the material you should see a 'normal editor' (like 'normal-editor.png').
6. Open 'myshader.shader' again, and uncomment CustomEditor "StandardShaderGUI"
7. Get back to Unity.

Results: Normal editor is not replaced by custom GUI (until you 'force' a refresh by clicking off the Cube and back on again).

Expected: Custom GUI appears straight away, when switching back to Unity Editor.

Reproduces with: 5.3.0f4; 5.4.0f3; 5.4.2f2; 5.5.0p1; 5.6.0b1