Add unit tests

• test_fetch_series_by_code
• test_fetch_series_by_code_mask
• test_fetch_series_by_code_mask_with_plus_in_dimension_code
• test_fetch_series_by_id
• test_fetch_series_by_ids_in_same_dataset
• test_fetch_series_by_ids_in_different_datasets
• test_fetch_series_by_dimension
• test_fetch_series_of_dataset
• test_fetch_series_by_api_link
• test_fetch_series_with_na_values
• test_fetch_series_with_max_nb_series
• test_fetch_series_with_filter_on_one_series
• test_fetch_series_with_filter_on_one_series_with_filter_error

(Source: test suite of the Python client (https://git.nomics.world/dbnomics/dbnomics-python-client/blob/master/tests/test_client.py))