Snapshot of ScaledParameter doesn't update when wrapped parameter is set

Steps to reproduce

1. Create ScaledParameter
2. Change value of wrapped parameter
3. Take snapshot of ScaledParameter. => It contains the old value

Expected behaviour

It is expected that also the snapshot of the ScaledParameter reflects the latest value of the wrapped behavior.

Actual behaviour

Snapshot of ScaledParameter is not updated.

System

Windows
qcodes release 0.21

Example to reproduce:

import qcodes as qc
from qcodes.tests.instrument_mocks import DummyInstrument
from pprint import pprint

iv_converter = DummyInstrument('iv_converter', gates=('adc1',))

current1 = qc.ScaledParameter(output=iv_converter.adc1, gain=1/1e6, name='current1', unit='A')

# This works fine
current1(4e-6)
pprint(current1.snapshot()) # this snapshot is correct

# Set adc1 to 3
iv_converter.adc1(3)
pprint(current1.snapshot()) # this snapshot  "value" is not correct (old value)

Start with ScaledParameter, its snapshot() method, and the wrapped DummyInstrument adc1 parameter shown in the reproduction. Reproduce the sequence of setting current1, changing adc1, and taking another snapshot; done means the second ScaledParameter snapshot reflects the wrapped parameter's latest value.