**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
}
    :type encryption_configuration: dict
    :param location: [Optional] The geographic location of the job. Required except for US and EU.
        See details at https://cloud.google.com/bigquery/docs/locations#specifying_your_location
    :type location: str
    :param impersonation_chain: Optional service account to impersonate using short-term
        credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.
        If set as a string, the account must grant the originating account
        the Service Account Token Creator IAM role.
        If set as a sequence, the identities from the list must grant
        Service Account Token Creator IAM role to the directly preceding identity, with first
        account from the list granting this role to the originating account (templated).
    :type impersonation_chain: Union[str, Sequence[str]]
    :param labels: [Optional] Labels for the BiqQuery table.
    :type labels: dict
    :param description: [Optional] Description for the BigQuery table.
    :type description: str
    """
    template_fields = (
        'bucket',
        'source_objects',
        'schema_object',
        'destination_project_dataset_table',
        'impersonation_chain',
    )
    template_ext = ('.sql',)
    ui_color = '#f0eee4'
    
    def __init__(