Improving jsonutil tests

[andrehora]

This PR improves the tests of jsonutil.py. It adds tests to parse_date and extract_dates by covering some untested cases. Coverage has also slightly increased.

Overall, I created 5 tests to cover some common and boundary cases:

• test_parse_date_invalid
• test_parse_date_valid
• test_parse_date_from_naive
• test_extract_dates_from_str
• test_extract_dates_from_dict

Also, the test test_extract_dates was renamed to test_extract_dates_from_list.

[blink1073]

Thanks very much!