**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
extra values that are not represented in the table schema.
        If true, the extra values are ignored. If false, records with extra columns
        are treated as bad records, and if there are too many bad records, an
        invalid error is returned in the job result.
    :type ignore_unknown_values: bool
    :param allow_quoted_newlines: Whether to allow quoted newlines (true) or not (false).
    :type allow_quoted_newlines: bool
    :param allow_jagged_rows: Accept rows that are missing trailing optional columns.
        The missing values are treated as nulls. If false, records with missing trailing
        columns are treated as bad records, and if there are too many bad records, an
        invalid error is returned in the job result. Only applicable to CSV, ignored
        for other formats.
    :type allow_jagged_rows: bool
    :param encoding: The character encoding of the data. See:
        https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs#configuration.query.tableDefinitions.(key).csvOptions.encoding
        https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#externalDataConfiguration.csvOptions.encoding
    :param max_id_key: If set, the name of a column in the BigQuery table
        that's to be loaded. This will be used to select the MAX value from
        BigQuery after the load occurs. The results will be returned by the
        execute() command, which in turn gets stored in XCom for future
        operators to use. This can be helpful with incremental loads--during
        future executions, you can pick up from the max ID.
    :type max_id_key: str
    :param bigquery_conn_id: (Optional) The connection ID used to connect to Google Cloud and
        interact with the BigQuery service.
    :type bigquery_conn_id: str
    :param google_cloud_storage_conn_id: (Optional) The connection ID used to connect to Google Cloud
        and interact with the Google Cloud Storage service.
    :type google_cloud_storage_conn_id: str
    :param delegate_to: The account to impersonate using domain-wide delegation of authority,