Fix validation for resource fields using preset datetime_tz

[solita-ijunnone]

Validation for resource fields using the preset datetime_tz does not work. For datasets the validation works fine.

Let's say I have a field called start_time under a resource using the preset datetime_tz. Submitting this field (or more accurately the three fields for date, time and tz which it gets split into) from the form will result in three different fields being saved in the database under extras: start_time_time, start_time_date and start_time_tz. This happens because the keys for resources are different in the data passed from the form to the validator method compared to the keys used for datasets.

This pull request contains a fix to the validator as well as a test case. Please make suggestions or corrections if there is a better or more appropriate way to solve this issue.