Supports nullable unique constraints

[shlee322]

Supports nullable unique constraints (optional). Ref #43

Example

DATABASES = {
    'default': {
        'ENGINE': 'sql_server.pyodbc',
        'NAME': 'mydb',
        'USER': 'user@myserver',             
        'PASSWORD': 'password',
        'HOST': 'myserver.database.windows.net',
        'PORT': '',

        'OPTIONS': {
            'driver': 'ODBC Driver 11 for SQL Server',
            'supports_nullable_unique_constraints': True
        },
    },
}

[kevin-brown]

Wouldn't it make more sense to just check connection.settings_dict['options'] within DatabaseFeatures.__init__? That way you don't need to change the signature around for every custom feature that's implemented.

[shlee322]

@kevin-brown
i just wanted to overwirte BaseDatabaseFeatures.supports_nullable_unique_constraint

how would you think about :

self.features = DatabaseFeatures(self)
self.features.supports_nullable_unique_constraints = opts.get('supports_nullable_unique_constraints', False)