Change the protect_from_forgery prepend default to false

Per this comment
https://github.com/rails/rails/pull/18334#issuecomment-69234050 we want
`protect_from_forgery` to default to `prepend: false`.

`protect_from_forgery` will now be insterted into the callback chain at the
point it is called in your application. This is useful for cases where you
want to `protect_from_forgery` after you perform required authentication
callbacks or other callbacks that are required to run after forgery protection.

If you want `protect_from_forgery` callbacks to always run first, regardless of
position they are called in your application, then you can add `prepend: true`
to your `protect_from_forgery` call.

Example:

```ruby
protect_from_forgery prepend: true
```

actionpack/CHANGELOG.md

@@ -1,3 +1,26 @@
+*   Change the `protect_from_forgery` prepend default to `false`
+
+    Per this comment
+    https://github.com/rails/rails/pull/18334#issuecomment-69234050 we want
+    `protect_from_forgery` to default to `prepend: false`.
+
+    `protect_from_forgery` will now be insterted into the callback chain at the
+    point it is called in your application. This is useful for cases where you
+    want to `protect_from_forgery` after you perform required authentication
+    callbacks or other callbacks that are required to run after forgery protection.
+
+    If you want `protect_from_forgery` callbacks to always run first, regardless of
+    position they are called in your application then you can add `prepend: true`
+    to your `protect_from_forgery` call.
+
+    Example:
+
+    ```ruby
+    protect_from_forgery prepend: true
+    ```
+
+    * Eileen M. Uchitelle*
+
 *   In url_for, never append a question mark to the URL when the query string
     is empty anyway.  (It used to do that when called like `url_for(controller:
     'x', action: 'y', q: {})`.)

actionpack/lib/action_controller/metal/request_forgery_protection.rb

@@ -102,13 +102,13 @@ module ActionController #:nodoc:
       #
       # Valid Options:
       #
-      # * <tt>:only/:except</tt> - Only apply forgery protection to a subset of actions. Like <tt>only: [ :create, :create_all ]</tt>.
+      # * <tt>:only/:except</tt> - Only apply forgery protection to a subset of actions. For example <tt>only: [ :create, :create_all ]</tt>.
       # * <tt>:if/:unless</tt> - Turn off the forgery protection entirely depending on the passed Proc or method reference.
-      # * <tt>:prepend</tt> - By default, the verification of the authentication token is added to the front of the
+      # * <tt>:prepend</tt> - By default, the verification of the authentication token will be added at the position of the
-      #   callback chain. If you need to make the verification depend on other callbacks, like authentication methods
+      #    protect_from_forgery call in your application. This means any callbacks added before are run first. This is useful
-      #   (say cookies vs OAuth), this might not work for you. Pass <tt>prepend: false</tt> to just add the
+      #    when you want your forgery protection to depend on other callbacks, like authentication methods (Oauth vs Cookie auth).
-      #   verification callback in the position of the protect_from_forgery call. This means any callbacks added
+      #
-      #   before are run first.
+      #    If you need to add verification to the beginning of the callback chain, use <tt>prepend: true</tt>.
       # * <tt>:with</tt> - Set the method to handle unverified request.
       #
       # Valid unverified request handling methods are:
@@ -116,7 +116,7 @@ module ActionController #:nodoc:
       # * <tt>:reset_session</tt> - Resets the session.
       # * <tt>:null_session</tt> - Provides an empty session during request but doesn't reset it completely. Used as default if <tt>:with</tt> option is not specified.
       def protect_from_forgery(options = {})
-        options = options.reverse_merge(prepend: true)
+        options = options.reverse_merge(prepend: false)
 
         self.forgery_protection_strategy = protection_method_class(options[:with] || :null_session)
         self.request_forgery_protection_token ||= :authenticity_token

actionpack/test/controller/request_forgery_protection_test.rb

@@ -540,10 +540,10 @@ class PrependProtectForgeryBaseControllerTest < ActionController::TestCase
     assert_equal(expected_callback_order, @controller.called_callbacks)
   end
 
-  def test_verify_authenticity_token_is_prepended_by_default
+  def test_verify_authenticity_token_is_not_prepended_by_default
     @controller = PrependDefaultController.new
     get :index
-    expected_callback_order = ["verify_authenticity_token", "custom_action"]
+    expected_callback_order = ["custom_action", "verify_authenticity_token"]
     assert_equal(expected_callback_order, @controller.called_callbacks)
   end
 end