Convert a call to the deprecated _SQLLimit function to Limit

While 4f61730 removed all existant calls to _SQLLimit at the time,
d43c6cf was later merged, which added another.  Convert it to using
->Limit.
Do not assume that FIELD is passed to Limit

Multiple places assume that FIELD is defined; however, RT::Tickets'
_IntLimit uses the new-ish FUNCTION argument instead, which elicits a
number of warnings.  Avoid them by only examining case-sensitivity if
FIELD is defined.
Merge branch '4.2/autocomplete-ticket-warnings'