fix Char defaults in types.py

```python
from rootpy.types import CharCol
from rootpy.tree import Tree, TreeModel
from rootpy.io import root_open

class Event(TreeModel):
    char = CharCol()

with root_open("test.root", "recreate"):
    tree = Tree("test", model=Event)
    for i in xrange(10):
        tree.char = i
        tree.Fill()
    tree.Write()
```