Fixing an edge case when using objects as constraints

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.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -50,7 +50,19 @@ def serve(req)
 
         private
           def constraint_args(constraint, request)
-            constraint.arity == 1 ? [request] : [request.path_parameters, request]
+            arity = if constraint.respond_to?(:arity)
+              constraint.arity
+            else
+              constraint.method(:call).arity
+            end
+
+            if arity < 1
+              []
+            elsif arity == 1
+              [request]
+            else
+              [request.path_parameters, request]
+            end
           end
       end
 

actionpack/test/dispatch/routing_test.rb

@@ -115,6 +115,21 @@ def test_redirect_with_passing_constraint
     assert_equal 301, status
   end
 
+  def test_accepts_a_constraint_object_responding_to_call
+    constraint = Class.new do
+      def call(*); true; end
+      def matches?(*); false; end
+    end
+
+    draw do
+      get "/", to: "home#show", constraints: constraint.new
+    end
+
+    assert_nothing_raised do
+      get "/"
+    end
+  end
+
   def test_namespace_with_controller_segment
     assert_raise(ArgumentError) do
       draw do