[BUG] Router doesn't match parametrized path

Doesn't work
```r
rt = RestRserve:::Router$new()
rt$add_path("/{p1}/{p2}", match = "regex", id = '1')
rt$match_path("/a/b")

```
> NULL

Works
```r
rt$add_path("/p/{p1}/{p2}", match = "regex", id = '1')
rt$match_path("/p/a/b")
```
>[1] "1"
attr(,"parameters_path")
attr(,"parameters_path")$p1
[1] "a"
attr(,"parameters_path")$p2
[1] "b"