Allow using ICompositeUserType for collection element mappings in Mapping By Code

[martin-the-geek]

Attempting to map a collection of a composite user-type with mapping by code is currently rejected with an exception "Expected type implementing IUserType or IType."

The composite user type is allowed when mapping by xml.

It should be allowed with mapping by code too.

[fredericDelaporte]

Also done for map-key and key-property, see #1896.