unable to explicitly use element coder in CombiningValueStateSpec constructor

looks like the example:
 COUNT_STATE = CombiningValueStateSpec('count',
VarIntCoder(),
combiners.SumCombineFn())
in [https://beam.apache.org/blog/timely-processing/](https://beam.apache.org/blog/timely-processing/) doesn't work because of coder error.
```

Error message from worker: generic::unknown: Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py",
line 256, in _execute
response = task()
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py",
line 313, in
lambda: self.create_worker().do_instruction(request), request)
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py",
line 483, in do_instruction
getattr(request, request_type), request.instruction_id)
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py",
line 518, in process_bundle
bundle_processor.process_bundle(instruction_id))
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py",
line 983, in process_bundle
op.finish()
File "apache_beam/runners/worker/operations.py", line
697, in apache_beam.runners.worker.operations.DoOperation.finish
File "apache_beam/runners/worker/operations.py",
line 699, in apache_beam.runners.worker.operations.DoOperation.finish
File "apache_beam/runners/worker/operations.py",
line 702, in apache_beam.runners.worker.operations.DoOperation.finish
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py",
line 775, in commit
state.commit()
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py",
line 492, in commit
self._underlying_bag_state.commit()
File "/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py",
line 553, in commit
self._state_key, self._value_coder.get_impl(), self._added_elements)
File
"/usr/local/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py", line 1012, in extend

coder.encode_to_stream(element, out, True)
File "apache_beam/coders/coder_impl.py", line 777,
in apache_beam.coders.coder_impl.VarIntCoderImpl.encode_to_stream
File "apache_beam/coders/coder_impl.py",
line 779, in apache_beam.coders.coder_impl.VarIntCoderImpl.encode_to_stream
TypeError: an integer is
required

```

The actual element type here was a list of int, not just int.

The API document mentions that
```

coder (Coder): Coder specifying how to encode the values to be combined.
May be inferred.

```

which is seemingly wrong.

Imported from Jira [BEAM-10696](https://issues.apache.org/jira/browse/BEAM-10696). Original Jira may contain additional context.
Reported by: heejong.