Python SDK generates requests to BigQuery API's method Tabledata:list with invalid argument alt

When stream inserts to BigQuery is involved, in io.gcp.bigquery.start_bundle, checking if table is empty generates requests to the [Tabledata:list method](https://cloud.google.com/bigquery/docs/reference/rest/v2/tabledata/list) of BigQuery API. Problem is it includes a parameter alt=json to the request, which doesn't seem accepted by the API.

Original log:

` java.util.concurrent.ExecutionException: java.lang.RuntimeException: Error received from SDK harness for instruction -4955710: Traceback (most recent call last):`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 131, in _execute`
`    response = task()`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 166, in `
`    self._execute(lambda: worker.do_instruction(work), work)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 212, in do_instruction`
`    request.instruction_id)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 234, in process_bundle`
`    processor.process_bundle(instruction_id)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/bundle_processor.py", line 404, in process_bundle`
`    op.start()`
`  File "apache_beam/runners/worker/operations.py", line 368, in apache_beam.runners.worker.operations.DoOperation.start`
`    def start(self):`
`  File "apache_beam/runners/worker/operations.py", line 369, in apache_beam.runners.worker.operations.DoOperation.start`
`    with self.scoped_start_state:`
`  File "apache_beam/runners/worker/operations.py", line 420, in apache_beam.runners.worker.operations.DoOperation.start`
`    self.dofn_runner.start()`
`  File "apache_beam/runners/common.py", line 695, in apache_beam.runners.common.DoFnRunner.start`
`    self._invoke_bundle_method(self.do_fn_invoker.invoke_start_bundle)`
`  File "apache_beam/runners/common.py", line 692, in apache_beam.runners.common.DoFnRunner._invoke_bundle_method`
`    self._reraise_augmented(exn)`
`  File "apache_beam/runners/common.py", line 702, in apache_beam.runners.common.DoFnRunner._reraise_augmented`
`    raise`
`  File "apache_beam/runners/common.py", line 690, in apache_beam.runners.common.DoFnRunner._invoke_bundle_method`
`    bundle_method()`
`  File "apache_beam/runners/common.py", line 348, in apache_beam.runners.common.DoFnInvoker.invoke_start_bundle`
`    def invoke_start_bundle(self):`
`  File "apache_beam/runners/common.py", line 352, in apache_beam.runners.common.DoFnInvoker.invoke_start_bundle`
`    self.signature.start_bundle_method.method_value())`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/io/gcp/bigquery.py", line 1359, in start_bundle`
`    self.create_disposition, self.write_disposition)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/utils/retry.py", line 184, in wrapper`
`    return fun(*args, **kwargs)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/io/gcp/bigquery.py", line 1100, in get_or_create_table`
`    table_empty = self._is_table_empty(project_id, dataset_id, table_id)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/utils/retry.py", line 184, in wrapper`
`    return fun(*args, **kwargs)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/io/gcp/bigquery.py", line 972, in _is_table_empty`
`    response = self.client.tabledata.List(request)`
`  File "/usr/local/lib/python2.7/dist-packages/apache_beam/io/gcp/internal/clients/bigquery/bigquery_v2_client.py", line 500, in List`
`    config, request, global_params=global_params)`
`  File "/usr/local/lib/python2.7/dist-packages/apitools/base/py/base_api.py", line 722, in _RunMethod`
`    return self.ProcessHttpResponse(method_config, http_response, request)`
`  File "/usr/local/lib/python2.7/dist-packages/apitools/base/py/base_api.py", line 728, in ProcessHttpResponse`
`    self.__ProcessHttpResponse(method_config, http_response, request))`
`  File "/usr/local/lib/python2.7/dist-packages/apitools/base/py/base_api.py", line 599, in __ProcessHttpResponse`
`    http_response, method_config=method_config, request=request)`
`HttpBadRequestError: HttpError accessing : response: <\{'status': '400', 'content-length': '245', 'x-xss-protection': '1; mode=block', 'transfer-encoding': 'chunked', 'vary': 'Origin, X-Origin, Referer', 'server': 'ESF', '-content-encoding': 'gzip', 'cache-control': 'private', 'date': 'Sun, 09 Dec 2018 06:16:01 GMT', 'x-frame-options': 'SAMEORIGIN', 'content-type': 'application/json; charset=UTF-8'}>, content <{`
`  "error": {`
`    "code": 400,`
`    "message": "Request contains an invalid argument.",`
`    "errors": [`
`      {`
`        "message": "",`
`        "domain": "global",`
`        "reason": "invalid"`
`      `}
`    ],`
`    "status": "INVALID_ARGUMENT"`
`  `}
``}
`>`

Imported from Jira [BEAM-6198](https://issues.apache.org/jira/browse/BEAM-6198). Original Jira may contain additional context.
Reported by: tpilewicz.