FIX #2174: SearchFilter needs casting helper for DataObject base fields

Commit 964b3f2 fixed an issue where dbObject was returning casting helpers for
fields that were not actually DB objects, but had something in $casting config.

However, because dbObject was no longer calling DataObject->castingHelper, this
exposed a bug that the underlying function db($fieldName) was not returning
field specs for the base fields that are created by SS automatically on all
DataObjects (i.e. Created, LastEdited, etc).

This commit fixes the underlying issue that DataObject->db($fieldName) should
return the field specs for *all* DB fields like its documentation says it will,
including those base fields that are automatically created and do not appear in
$db.
Merge pull request #2175 from jthomerson/fix_2174

FIX #2174: SearchFilter needs casting helper for DataObject base fields