**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
#
#
#
"""This module contains a Google Cloud Storage to BigQuery operator."""
import json
from typing import Optional, Sequence, Union
from airflow.exceptions import AirflowException
from airflow.models import BaseOperator
from airflow.providers.google.cloud.hooks.bigquery import BigQueryHook
from airflow.providers.google.cloud.hooks.gcs import GCSHook
class GCSToBigQueryOperator(BaseOperator):