Fix Guides to include #23138 [ci skip]

A long-standing bug has been fixed in Rails 5.1 by @pixeltrix
See https://github.com/rails/rails/blame/5-1-stable/actionpack/CHANGELOG.md#L52-L64

This PR updates the guides accordingly.

Before:
<img width="799" alt="before" src="https://cloud.githubusercontent.com/assets/10076/24809105/4ec7066c-1b73-11e7-98c6-c709c3465306.png">

After:
<img width="799" alt="after" src="https://cloud.githubusercontent.com/assets/10076/24809109/50b21188-1b73-11e7-8a1a-32c3d0ed512a.png">

guides/source/routing.md

@@ -152,6 +152,7 @@ This resourceful route:
 
 ```ruby
 resource :geocoder
+resolve('Geocoder') { [:geocoder] }
 ```
 
 creates six different routes in your application, all mapping to the `Geocoders` controller:
@@ -175,14 +176,6 @@ A singular resourceful route generates these helpers:
 
 As with plural resources, the same helpers ending in `_url` will also include the host, port and path prefix.
 
-WARNING: A [long-standing bug](https://github.com/rails/rails/issues/1769) prevents `form_for` from working automatically with singular resources. As a workaround, specify the URL for the form directly, like so:
-
-```ruby
-form_for @geocoder, url: geocoder_path do |f|
-
-# snippet for brevity
-```
-
 ### Controller Namespaces and Routing
 
 You may wish to organize groups of controllers under a namespace. Most commonly, you might group a number of administrative controllers under an `Admin::` namespace. You would place these controllers under the `app/controllers/admin` directory, and you can group them together in your router: