'Calculate the signature from a dictionary of params.
NOTE: This method uses the processor\'s hashing method.  That method
is a thin wrapper of standard library calls, and it seemed overly complex
to rewrite that code in the test suite.
Args:
params (dict): Dictionary with a key \'signed_field_names\',
which is a comma-separated list of keys in the dictionary
to include in the signature.
Returns:
string'

def _signature(self, params):

    return processor_hash(','.join([u'{0}={1}'.format(signed_field, params[signed_field]) for signed_field in params['signed_field_names'].split(u',')]))
