Do not recursively call public PlanScan that caches

Otherwise, wrapper types get cached. Wrapper types are expected to fail
most of the time. These failures should not be cached. In addition,
wrappers wrap multiple different types so it doesn't make sense to cache
results of a wrapper.

pgtype/pgtype.go

@@ -1068,7 +1068,7 @@ func (m *Map) planScan(oid uint32, formatCode int16, target interface{}) ScanPla
 
 	for _, f := range m.TryWrapScanPlanFuncs {
 		if wrapperPlan, nextDst, ok := f(target); ok {
-			if nextPlan := m.PlanScan(oid, formatCode, nextDst); nextPlan != nil {
+			if nextPlan := m.planScan(oid, formatCode, nextDst); nextPlan != nil {
 				if _, failed := nextPlan.(*scanPlanFail); !failed {
 					wrapperPlan.SetNext(nextPlan)
 					return wrapperPlan