Add a new test to test_agency: an exception is thrown if agency_url field is missing.
Agency refactoring: add a new method _mandatoryFieldIsMissing to Agency,
and a method hasField(fieldName) to FieldParder.
Add a new test to test_agency: an exception is thrown if agency_timezone is missing.

Fixed a type in another test.
Replace the full strings in Agency._mandatoryFieldIsMissing() with
the static consts.