**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
https://cloud.google.com/bigquery/docs/reference/v2/jobs#configuration.load
        Should not be set when source_format is 'DATASTORE_BACKUP'.
        Parameter must be defined if 'schema_object' is null and autodetect is False.
    :type schema_fields: list
    :param schema_object: If set, a GCS object path pointing to a .json file that
        contains the schema for the table. (templated)
        Parameter must be defined if 'schema_fields' is null and autodetect is False.
    :type schema_object: str
    :param source_format: File format to export.
    :type source_format: str
    :param compression: [Optional] The compression type of the data source.
        Possible values include GZIP and NONE.
        The default value is NONE.
        This setting is ignored for Google Cloud Bigtable,
        Google Cloud Datastore backups and Avro formats.
    :type compression: str
    :param create_disposition: The create disposition if the table doesn't exist.
    :type create_disposition: str
    :param skip_leading_rows: Number of rows to skip when loading from a CSV.
    :type skip_leading_rows: int
    :param write_disposition: The write disposition if the table already exists.
    :type write_disposition: str
    :param field_delimiter: The delimiter to use when loading from a CSV.
    :type field_delimiter: str
    :param max_bad_records: The maximum number of bad records that BigQuery can
        ignore when running the job.
    :type max_bad_records: int
    :param quote_character: The value that is used to quote data sections in a CSV file.
    :type quote_character: str
    :param ignore_unknown_values: [Optional] Indicates if BigQuery should allow