Invalid behaviour of accept_scalar for str in Python 3.

In this issue https://github.com/Pylons/colander/issues/81 was mentioned example:
```Python
import colander


class DatesSchema(colander.SequenceSchema):
    date = colander.SchemaNode(colander.Date())

if __name__ == '__main__':
    schema = DatesSchema(colander.Sequence(accept_scalar=True))
    data = schema.deserialize(['2013-02-14', '2013-02-15'])
    print data
    # [datetime.date(2013, 2, 14), datetime.date(2013, 2, 15)]
    data = schema.deserialize('2013-02-14')
    print data
    # [datetime.date(2013, 2, 14)]
```
This code works fine in Python 2.7 but in Python 3.3 it returns:
```Python
colander.Invalid: {'0': 'Invalid date',
 '1': 'Invalid date',
 '2': 'Invalid date',
 '3': 'Invalid date',
 '4': 'Invalid date',
 '5': 'Invalid date',
 '6': 'Invalid date',
 '7': 'Invalid date',
 '8': 'Invalid date',
 '9': 'Invalid date'}
```
The following occurs: colander represent string like a iterable type and put it into list().
Check that value is not a string type and tests for it was added.