Fix route dispatcher extension stripping behavior

The negotiation process extension behavior is misbehaving.

With a such route : 
```csharp
Get ["/{filename*}"] = parameters => {
                return parameters.filename;
           '};
```

When trying to get `/directory.xmls/file.xml` the route is rewritten to `/directorys/file` instead of `/directory.xmls/file`, this patch should fix this.