Add convert method to the Phalcon\Mvc\Router\RouteInterface

[sergeyklay]

The documentation for Phalcon\Mvc\Router\Group::add states, that add() on a group would return an instance of RouteInterface but it really returns an instance of Route.

After calling add we shouldn't call convert because convert is a Route member but not RouteInterface.

[sergeyklay]

Implemented in the 4.0.x branch.