Use db_to_form_package_schema() by default

Make DefaultDatasetForm's db_to_form_schema() method return
db_to_form_package_schema() rather than None, so that validation against
db_to_form_package_schema() is done by default, even when there's no
IDatasetForm plugin active.

The reason for this is that bugs in db_to_form_package_schema() can
cause pages to crash, features to stop working, etc., but the tests
currently do not catch these bugs because db_to_form_package_schema() is
only used when an IDatasetForm plugin that returns a schema based on
db_to_form_package_schema() is active, and no such plugin is active when
the tests run.

ckan/lib/plugins.py

@@ -251,6 +251,7 @@ def form_to_db_schema_api_update(self):
     def db_to_form_schema(self):
         '''This is an interface to manipulate data from the database
         into a format suitable for the form (optional)'''
+        return logic.schema.db_to_form_package_schema()
 
     def db_to_form_schema_options(self, options):
         '''This allows the selectino of different schemas for different