support function pointer in match method of URLRouter

this is to support the following code:

```
import vibe.d;

void index(HTTPServerRequest req, HTTPServerResponse res){}

shared static this()
{
	auto router = new URLRouter;
	router.match(HTTPMethod.OPTIONS,"*", &index);

	listenHTTP(new HTTPServerSettings, router);
}
```

without this change the compiler refuses with:
```
source\app.d(8): Error: function vibe.http.router.URLRouter.match (HTTPMethod method, string path, v
oid delegate(HTTPServerRequest req, HTTPServerResponse res) cb) is not callable using argument types
 (HTTPMethod, string, void function(HTTPServerRequest req, HTTPServerResponse res))
```

is this maybe even a compiler bug ??