'Performs a check on the data within the bundle (and optionally the
request) to ensure it is valid.
Should return a dictionary of error messages. If the dictionary has
zero items, the data is considered valid. If there are errors, keys
in the dictionary should be field names and the values should be a list
of errors, even if there is only one.'

def is_valid(self, bundle, request=None):

    return {}
