Force given path to http methods in mapper to skip canonical action checking

This fixes the following scenario:

    resources :contacts do
      post 'new', action: 'new', on: :collection, as: :new
    end

Where the /new path is not generated because it's considered a canonical
action, part of the normal resource actions:

    new_contacts POST   /contacts(.:format)          contacts#new

Fixes #2999

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -535,7 +535,8 @@ def delete(*args, &block)
         private
           def map_method(method, args, &block)
             options = args.extract_options!
-            options[:via] = method
+            options[:via]    = method
+            options[:path] ||= args.first if args.first.is_a?(String)
             match(*args, options, &block)
             self
           end
@@ -1509,7 +1510,7 @@ def path_for_action(action, path) #:nodoc:
             prefix = shallow_scoping? ?
               "#{@scope[:shallow_path]}/#{parent_resource.path}/:id" : @scope[:path]
 
-            path = if canonical_action?(action, path.blank?)
+            if canonical_action?(action, path.blank?)
               prefix.to_s
             else
               "#{prefix}/#{action_path(action, path)}"

actionpack/test/controller/routing_test.rb

@@ -1405,7 +1405,7 @@ def test_route_constraints_with_supported_options_must_not_error
       end
     end
   end
-  
+
   def test_route_with_subdomain_and_constraints_must_receive_params
     name_param = nil
     set.draw do
@@ -1418,7 +1418,7 @@ def test_route_with_subdomain_and_constraints_must_receive_params
       set.recognize_path('http://subdomain.example.org/page/mypage'))
     assert_equal(name_param, 'mypage')
   end
-  
+
   def test_route_requirement_recognize_with_ignore_case
     set.draw do
       get 'page/:name' => 'pages#show',

actionpack/test/dispatch/routing_test.rb

@@ -171,6 +171,8 @@ def self.call(params, request)
             post :preview, :on => :collection
           end
         end
+
+        post 'new', :action => 'new', :on => :collection, :as => :new
       end
 
       resources :replies do
@@ -876,6 +878,12 @@ def test_projects
     assert_equal '/projects/1/edit', edit_project_path(:id => '1')
   end
 
+  def test_projects_with_post_action_and_new_path_on_collection
+    post '/projects/new'
+    assert_equal "project#new", @response.body
+    assert_equal "/projects/new", new_projects_path
+  end
+
   def test_projects_involvements
     get '/projects/1/involvements'
     assert_equal 'involvements#index', @response.body
@@ -2450,7 +2458,6 @@ def app; Routes end
     get "/foo/bar/baz"
     assert_equal "/pooh", @response.body
   end
-
 end
 
 class TestTildeAndMinusPaths < ActionDispatch::IntegrationTest