[BUG] Scaling histogram doesn't affect flow bins

[andrzejnovak]

def test_scale_flowbins():    
    w = 1e-2
    x = np.random.normal(loc=0.4, scale=0.4, size=1000)
    y = np.random.normal(loc=0.6, scale=0.4, size=1000)
    
    h = bh.Histogram(
        bh.axis.Regular(3, 0, 1),
        bh.axis.Regular(3, 0, 1),
        storage=bh.storage.Weight(),
    )
    
    h.fill(x, y, weight=w)

    ref_value = np.array([h[bh.underflow,  0].value, h[1, 0].value, h[bh.overflow,  0].value])
    h = h * 5
    scale_value = np.array([h[bh.underflow,  0].value, h[1, 0].value, h[bh.overflow,  0].value])
    return np.allclose(ref_value * 5 , scale_value)

[henryiii]

def test_scale_flowbins():
    w = 1e-1
    x = np.random.normal(loc=0.4, scale=0.4, size=100)

    h = bh.Histogram(
        bh.axis.Variable([0, 0.5, 1]),
        storage=bh.storage.Weight()
        )

    h.fill(x, weight=w)

    ref_value = h.values(flow=True) * 5
    scale_value = (h * 5).values(flow=True)

    assert scale_value == approx(ref_value)