fix the nil judge

@@ -22,7 +22,7 @@ First you must import it
 
 Then init a Cache (example with memory adapter)
 
-	bm, err := NewCache("memory", `{"interval":60}`)	
+	bm, err := cache.NewCache("memory", `{"interval":60}`)	
 
 Use it like this:	
 	

@@ -131,7 +131,7 @@ func getFieldType(val reflect.Value) (ft int, err error) {
 	case reflect.String:
 		ft = TypeCharField
 	default:
-		if elm == nil {
+		if elm.IsNil() {
 			panic(fmt.Errorf("%s is nil pointer, may be miss setting tag", val))
 		}
 		switch elm.Interface().(type) {