**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
allow_quoted_newlines=False,
        allow_jagged_rows=False,
        encoding="UTF-8",
        max_id_key=None,
        bigquery_conn_id='google_cloud_default',
        google_cloud_storage_conn_id='google_cloud_default',
        delegate_to=None,
        schema_update_options=(),
        src_fmt_configs=None,
        external_table=False,
        time_partitioning=None,
        cluster_fields=None,
        autodetect=True,
        encryption_configuration=None,
        location=None,
        impersonation_chain: Optional[Union[str, Sequence[str]]] = None,
        labels=None,
        description=None,
        **kwargs,
    ):
        super().__init__(**kwargs)
        
        if src_fmt_configs is None:
            src_fmt_configs = {}
        if time_partitioning is None:
            time_partitioning = {}
        self.bucket = bucket
        self.source_objects = source_objects
        self.schema_object = schema_object