IECoreMaya : Fixed failing ParameterisedHolder unit test.
IECoreMaya : Fixed failing ParameterisedHolder unit test.

The test was failing for two reasons:
1. The meshMerge op could not be found
2. `IECoreScene::MeshPrimitive::createBox` uses indexed normals.
   However, ToMayaMeshConverter was not correctly converting them,
   and then when the scene was reloaded maya was automatically
   recalculating them.

appleseedhq/cortex,appleseedhq/cortex,appleseedhq/cortex