FIX: <% if Link %> wasn't working

Since ViewableData was returning a casting helper for Link, but DataObject was
only using $this->$fieldname to set values on that casting helper, you could
not use <% if Link %> (or <% if $Link %>) in your templates because Link is not
a field, and thus had no value to be set on the casting helper, causing
hasValue to think that there was no value.  Since DataObject->dbObject says that
"it only matches fields and not methods", it seems safe to have it call db(..)
to get the field spec, and not call ViewableData->castingHelper at all.
Merge pull request #2021 from jthomerson/fix_if_link_not_working

FIX: <% if Link %> wasn't working