add optional 3rd arg in operations.field_case_sql()

+ neither internal_type nor db_type are used anywhere in field_case_sql function
+ complies with both django 1.6 API (3 args) and Django 1.5 (2 args)
+ patch proposed by christian@bachand-tremblay.com on https://github.com/lionheart/django-pyodbc/issues/33
+ tested on Django 1.5 and 1.6 with FreeTDS and pyodbc 3.0.7 by HL on a fc16 box communicating with sqlserver 2008