Dynamic Range does not call _default method

Running
```
from traits.api import HasTraits, Int, Range

class Foo(HasTraits):

bar = Int(10)

baz = Range(low=0, high=10)

def _baz_default(self):
print('_baz_default')
return 5

if __name__ == '__main__':
foo = Foo()
print(foo.baz)
```
yields
```
>>> _baz_default
>>> 5
```

but changing the Range trait definition to `baz = Range(low=0, high="bar")`

the output is
```
>>> 0
```