`Union` doesn't handle mapped traits

[corranwebster]

An issue we are currently seeing trying to remove Trait(...) and Either(...) from Chaco: when you have Union(..., FooTrait, ...) where FooTrait is a mapped trait, union does not carry over the mapping behaviour, where Trait(...) did (I have not checked exactly what it does). There may be a similar issue with post_setattr?

I'm not sure what the right fix is in this case: you could say that a Union with any mapped trait is a mapped trait, but what do you set into the trait for un-mapped values: the safest is probably just to pass the value through, but arguments could be made for None or Undefined or similar.