fix zero type

pmap.go

@@ -112,7 +112,7 @@ func (n collision) getNode(hash uint32, key Key) interface{} {
 	return nil
 }
 
-func lookup(root interface{}, shift, hash uint32, key Key, zero Key) (Value, bool) {
+func lookup(root interface{}, shift, hash uint32, key Key, zero Value) (Value, bool) {
 	cur := root
 	for {
 		switch n := cur.(type) {