support decimals in fixture attribute values

http://manage.dimagi.com/default.asp?133261
I think that will always result in an empty string. Did you mean to say:

`u'{}'.format(field_with_attr.properties[attribute] or '')`

An alternative would be

`unicode(field_with_attr.properties[attribute] or '')`

Either of these will return an empty string if the value of the property is zero (0) or any other falsy value. A more defensive approach would be:

```py
from decimal import Decimal
...
    for attribute in field_with_attr.properties:
        val = field_with_attr.properties[attribute]
        xField.attrib[attribute] = unicode(val) if isinstance(val, Decimal) else val
```