Overriding Trait definitions does not respect MRO

I don't think this is a duplicate; and this behaviour seems wrong:
```
from traits.api import HasTraits, Unicode

class A(HasTraits):
x = Unicode('test')

class B(A):
pass

class C(A):
x = Unicode('override')

class D(B, C):
pass

d = D()
print(d.x)
print(D.mro())
```
which gives result
```
test
[, , , , , , ]
```

I would expect that the default value from `C` would be used. Note that it does work as expected when overriding the definition in `B` or `D`.