Wrong PathParams and Group token idx on child Mux mount

## Issue

When creating a child Mux with PathParams:
```
m.Handle("sub.$id", handlerOpt);
```
Or with groups
```
m.Handle("sub.$id", handlerOpt, res.Group("my${id}group"));
```

The idx will be set relative to the subpath, which becomes wrong when mounted to a root.
This results in the wrong PathParams and the wrong Group name.