**Apache Airflow version**:1.10.12
**Environment**: google cloud composer

**What happened**:
When using GoogleCloudStorageToBigQueryOperator and providing string as parameter source_objects, the process is iterating on a the string as a valid list.
For example - 
`cloud_storage_to_bigquery = GoogleCloudStorageToBigQueryOperator(
        bucket = 'bucket',
        source_objects = 'abc',
    )`
Will result in looking into the sources: bucket/a, bucket/b, bucket/c.

**What you expected to happen**:
Throw an error on type (string instead of list). 

airflow/providers/google/cloud/transfers/gcs_to_bigquery.py
bq_hook = BigQueryHook(
            bigquery_conn_id=self.bigquery_conn_id,
            delegate_to=self.delegate_to,
            location=self.location,
            impersonation_chain=self.impersonation_chain,
        )
        if not self.schema_fields:
            if self.schema_object and self.source_format != 'DATASTORE_BACKUP':
                gcs_hook = GCSHook(
                    gcp_conn_id=self.google_cloud_storage_conn_id,
                    delegate_to=self.delegate_to,
                    impersonation_chain=self.impersonation_chain,
                )
                blob = gcs_hook.download(
                    bucket_name=self.bucket,
                    object_name=self.schema_object,
                )
                schema_fields = json.loads(blob.decode("utf-8"))
            elif self.schema_object is None and self.autodetect is False:
                raise AirflowException(
                    'At least one of `schema_fields`, `schema_object`, or `autodetect` must be passed.'
                )
            else:
                schema_fields = None
        else:
            schema_fields = self.schema_fields
        source_uris = [f'gs://{self.bucket}/{source_object}' for source_object in self.source_objects]
        conn = bq_hook.get_conn()
        cursor = conn.cursor()