**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
if any. For this to work, the service account making the request must have
        domain-wide delegation enabled.
    :type delegate_to: str
    :param schema_update_options: Allows the schema of the destination
        table to be updated as a side effect of the load job.
    :type schema_update_options: list
    :param src_fmt_configs: configure optional fields specific to the source format
    :type src_fmt_configs: dict
    :param external_table: Flag to specify if the destination table should be
        a BigQuery external table. Default Value is False.
    :type external_table: bool
    :param time_partitioning: configure optional time partitioning fields i.e.
        partition by field, type and  expiration as per API specifications.
        Note that 'field' is not available in concurrency with
        dataset.table$partition.
    :type time_partitioning: dict
    :param cluster_fields: Request that the result of this load be stored sorted
        by one or more columns. BigQuery supports clustering for both partitioned and
        non-partitioned tables. The order of columns given determines the sort order.
        Not applicable for external tables.
    :type cluster_fields: list[str]
    :param autodetect: [Optional] Indicates if we should automatically infer the
        options and schema for CSV and JSON sources. (Default: ``True``).
        Parameter must be set to True if 'schema_fields' and 'schema_object' are undefined.
        It is suggested to set to True if table are create outside of Airflow.
    :type autodetect: bool
    :param encryption_configuration: [Optional] Custom encryption configuration (e.g., Cloud KMS keys).
        **Example**: ::
            encryption_configuration = {
                "kmsKeyName": "projects/testp/locations/us/keyRings/test-kr/cryptoKeys/test-key"