**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
self,
        *,
        bucket,
        source_objects,
        destination_project_dataset_table,
        schema_fields=None,
        schema_object=None,
        source_format='CSV',
        compression='NONE',
        create_disposition='CREATE_IF_NEEDED',
        skip_leading_rows=0,
        write_disposition='WRITE_EMPTY',
        field_delimiter=',',
        max_bad_records=0,
        quote_character=None,
        ignore_unknown_values=False,