Fix #1111 route arity.

We cannot know the real arity because (padrino/sinatra) creates
unbounded methods like:

  proc { |a,p| unbound_method.bind(a).call(*p) }

So the arity is always 2.

The corret way to counsume a route using only params that
we need is using _ like:

  get '/foo/:id/:maybe/:not' do |id, _|
    do_some_with(id)
  end

/cc @padrino/core-members, @ortuna