Revert "[#406] Don't strip id's from extras in validate()"

This reverts commit ecfacbbc41d85bc4a83bd3ac008f185ec31b9185.

It's fine to strip IDs from extras, extra IDs are never useful or needed
by users. Any tests that are failing (or templates that are crashing,
etc.) by trying to read extra IDs should be fixed to not read extra IDs.
[#406] Fix some broken tests

These tests were failing when run with an IDatasetForm plugin active
because they're trying to read extra IDs, which they shouldn't do.
Change the tests to not read extra IDs, now they pass.

See commits ecfacbbc4 and e6acbd4bc645.