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

db_to_form_package_schema() (which is only used if an IDatasetForm
plugin is active) was stripping the "id" keys from package extras, which
then causes some tests to fail. Not sure if just the tests were
crashing, or if some ckan pages were also crashing. Anyway, this commit
means they don't get stripped anymore.

Fixes #406