Overridden method that appends a field's value except in the case of a value that is a
ModelEntity, in which case it checks the RtaStaticDataCache to see if it caches objects
that value's class, and if it does, only sends to the buffer "CACHED className SID".
Capture the trace to a String and put it in the output, either in __trace__ in the request
attributes, or in the case of an ajax call with a JSONObject output, in a __trace__ member
of the returned JSONObject.