check if str is file path instead of type strings

[monstercode]

When doing

ScenarioFactory("""
        accounts:
          - username: Think Geek
        users:
          - account_id: $account_1.id
        warehouses:
          - from_name: Test Warehouse
            account_id: $account_1.id
            from_address1: 55 W Railroad Ave
            from_address2: Building 4
            from_zip: 10923...

and tries to update, the string is interpreted as a path to a file

  def update(self, source):
        if isinstance(source, dict):
            raw = source
        else:
           raw = yaml.load(open(source) if isinstance(source, six.string_types) else source)
E           IOError: [Errno 36] File name too long: u"\n        accounts:\n          - username: Think Geek\n        users:\n          - account_id: $account_1.id\n        warehouses:\n          - from_name: Test Warehouse\n            account_id: $account_1.id\n            from_address1: 55 W Railroad Ave\n            from_address2: Building 4\n            from_zip: 10923\n            from_city: New York\n  

isinstance(source, six.string_types) == True