**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). 

tests/providers/google/cloud/transfers/test_gcs_to_bigquery.py
#
#
#
import unittest
from unittest import mock
from airflow.providers.google.cloud.transfers.gcs_to_bigquery import GCSToBigQueryOperator
TASK_ID = 'test-gcs-to-bq-operator'
TEST_EXPLICIT_DEST = 'test-project.dataset.table'
TEST_BUCKET = 'test-bucket'
MAX_ID_KEY = 'id'
TEST_SOURCE_OBJECTS = ['test/objects/*']
LABELS = {'k1': 'v1'}
DESCRIPTION = "Test Description"
class TestGoogleCloudStorageToBigQueryOperator(unittest.TestCase):