Fix upper default critic bug

Fix default locale to make a query with upper version.
Leaded to a bug when an unknown locale was passed to this method. Query
was .get('default') instead of .get('DEFAULT').

Example:
>>> from moneyed.localization import _FORMATTER
_FORMATTER.get_sign_definition('EUR', 'fr_FR')

Traceback (most recent call last):
  File "<input>", line 2, in <module>
  File "build\bdist.win-amd64\egg\moneyed\localization.py", line 46, in get_sign_definition
    if currency_code in local_set:
TypeError: argument of type 'NoneType' is not iterable