Check if a route is already defined when calling ->add_route.

When adding routes to an application, check if the route is already
defined for the given method.  If that's the case, log the error and
die.
Add a test to the ->route_exists feature.
Duplicate routes croak instead of log+die.
reflect Changes
Merge branch 'franckcuny-pr/check-duplicate-on-add-route' into devel