This PR fixes an issue when the following situation occurs.
If you define a class like this

    class MyConstraint
      def call(*args)
        # for some reason this is defined
      end

      def matches?(*args)
        # checking the args
      end
    end

and try to use it as a constraint

    get "/", to: "home#show", constraints: MyConstraint.new

if its `matches?` method returns `false` there will be an error for the
mapper will ask for the constraint arity, thinking it is a proc, lambda
or method.

This PR checks for the presence of the `arity` method on the constraint
calling it only if present, preventing the error while keeping the basic
behavior.