Override default_render's behavior with a block

In 0de4a23 the behavior when there is a missing template was changed to
not raise an error, but instead head :no_content.  This is a breaking
change and some gems rely on this happening.

To allow gems and other code to work around this, allow
`default_render` to take a block which, if provided, will
execute the contents of that block instead of doing the `head :no_content`.

actionpack/lib/action_controller/metal/implicit_render.rb

@@ -3,14 +3,29 @@ module ActionController
 
     include BasicImplicitRender
 
+    # Renders the template corresponding to the controller action, if it exists.
+    # The action name, format, and variant are all taken into account.
+    # For example, the "new" action with an HTML format and variant "phone" 
+    # would try to render the <tt>new.html+phone.erb</tt> template.
+    #
+    # If no template is found <tt>ActionController::BasicImplicitRender</tt>'s implementation is called, unless
+    # a block is passed. In that case, it will override the super implementation.
+    #
+    #   default_render do
+    #     head 404 # No template was found
+    #   end
     def default_render(*args)
       if template_exists?(action_name.to_s, _prefixes, variants: request.variant)
         render(*args)
+      else
+        if block_given?
+          yield(*args)
         else
           logger.info "No template found for #{self.class.name}\##{action_name}, rendering head :no_content" if logger
           super
         end
       end
+    end
 
     def method_for_action(action_name)
       super || if template_exists?(action_name.to_s, _prefixes)

actionpack/test/controller/mime/respond_to_test.rb

@@ -793,3 +793,24 @@ class RespondToControllerTest < ActionController::TestCase
     assert_equal "phone", @response.body
   end
 end
+
+class RespondToWithBlockOnDefaultRenderController < ActionController::Base
+  def show
+    default_render do
+      render text: 'default_render yielded'
+    end
+  end
+end
+
+class RespondToWithBlockOnDefaultRenderControllerTest < ActionController::TestCase
+  def setup
+    super
+    @request.host = "www.example.com"
+  end
+
+  def test_default_render_uses_block_when_no_template_exists
+    get :show
+    assert_equal "default_render yielded", @response.body
+    assert_equal "text/html", @response.content_type
+  end
+end