Trait default values are validated even when not ever used

``` Python
from IPython.utils.traitlets import *

class Foo(HasTraits):
    bar = Type(allow_none=False)

foo = Foo(bar=int)
```

yields `The 'bar' trait of a Foo instance must be a subclass of object, but a value of type 'NoneType' (i.e. None) was specified.`

The default value is validated while it is not needed. 
