Tuple defaults not properly evaluated.

Example from @johntyree: the default for `f.tup` should be `(0, array_of_some_kind)`. Instead it's `(0, weird_tuple)`.

```
#!/usr/bin/env python
# -*- coding: utf-8 -*-

import traits.api as ta

class F(ta.HasTraits):
arr = ta.Array(shape=(None,), dtype=float)
tup = ta.Tuple(ta.Int, ta.Array(shape=(None,), dtype=float))

f = F()
f.print_traits()

# Output
# arr: array([ 0.])
# tup: (0, (>> from traits.api import *
>>> my_list = List([1, 2, 3])
>>> class A(HasTraits):
... foo = my_list
... bar = my_list
... baz = Tuple(my_list, Int)
... spam = Tuple(Str, my_list)
...
>>> a = A()
>>> a.foo.append(4)
>>> a.bar
[1, 2, 3]
>>> a.baz
([1, 2, 3], 0)
>>> a.baz[0].append(4) # a.spam should *not* have been modified by this!
>>> a.baz
([1, 2, 3, 4], 0) # as expected
>>> a.spam
('', [1, 2, 3, 4]) # not as expected: correct output is ('', [1, 2, 3])
```