'Test the identity transfer function.'

def test_identity(self):

    z = []
    p = []
    k = 1.0
    (b, a) = zpk2tf(z, p, k)
    b_r = np.array([1.0])
    a_r = np.array([1.0])
    assert_array_equal(b, b_r)
    assert_(isinstance(b, np.ndarray))
    assert_array_equal(a, a_r)
    assert_(isinstance(a, np.ndarray))
