Don't do runtime delegation, delegate explicitly (#789)

* Don't do runtime delegation, delegate explicitly

If someone accidentally calls `super` instead of `object` inside a decorator,
all hell breaks loose.

```rb
class Post < ActiveRecord::Base
  def title
    "title"
  end
end

class PostDecorator < Draper::Decorator
  delegate_all

  def title
    super and "overriden title"
  end
end

post = PostDecorator.new(Post.new)
post.title #=> "overriden title"
post.title #=> "title"
```

This patch makes both `super` and `object` work. I'm not sure why
runtime delegation was added in the first place, but it seems very wrong
to add methods on the fly.

* Add documentation and more tests.

* Consistent line spacing in test.

README.md

@@ -496,7 +496,10 @@ end
 
 When your decorator calls `delegate_all`, any method called on the decorator not
 defined in the decorator itself will be delegated to the decorated object. This
-is a very permissive interface.
+includes calling `super` from within the decorator. A call to `super` from within
+the decorator will first try to call the method on the parent decorator class. If
+the method does not exist on the parent decorator class, it will then try to call
+the method on the decorated `object`. This is a very permissive interface.
 
 If you want to strictly control which methods are called within views, you can
 choose to only delegate certain methods from the decorator to the source model:

lib/draper/automatic_delegation.rb

@@ -2,12 +2,14 @@ module Draper
   module AutomaticDelegation
     extend ActiveSupport::Concern
 
-    # Delegates missing instance methods to the source object.
+    # Delegates missing instance methods to the source object. Note: This will delegate `super`
+    # method calls to `object` as well. Calling `super` will first try to call the method on
+    # the parent decorator class. If no method exists on the parent class, it will then try
+    # to call the method on the `object`.
     def method_missing(method, *args, &block)
       return super unless delegatable?(method)
 
-      self.singleton_class.delegate method
-      send(method, *args, &block)
+      object.send(method, *args, &block)
     end
 
     # Checks if the decorator responds to an instance method, or is able to

spec/draper/decorator_spec.rb

@@ -572,12 +572,43 @@ module Draper
           expect(decorator.hello_world).to be :delegated
         end
 
-        it "adds delegated methods to the decorator when they are used" do
-          decorator = Decorator.new(double(hello_world: :delegated))
+        it 'delegates `super` to parent class first' do
+          parent_decorator_class = Class.new(Decorator) do
+            def hello_world
+              "parent#hello_world"
+            end
+          end
 
-          expect(decorator.methods).not_to include :hello_world
-          decorator.hello_world
-          expect(decorator.methods).to include :hello_world
+          child_decorator_class = Class.new(parent_decorator_class) do
+            def hello_world
+              super
+            end
+          end
+
+          decorator = child_decorator_class.new(double(hello_world: 'object#hello_world'))
+          expect(decorator.hello_world).to eq 'parent#hello_world'
+        end
+
+        it 'delegates `super` to object if method does not exist on parent class' do
+          decorator_class = Class.new(Decorator) do
+            def hello_world
+              super
+            end
+          end
+
+          decorator = decorator_class.new(double(hello_world: 'object#hello_world'))
+          expect(decorator.hello_world).to eq 'object#hello_world'
+        end
+
+        it 'raises `NoMethodError` when `super` is called on for method that does not exist' do
+          decorator_class = Class.new(Decorator) do
+            def hello_world
+              super
+            end
+          end
+
+          decorator = decorator_class.new(double)
+          expect{decorator.hello_world}.to raise_error NoMethodError
         end
 
         it "allows decorator to decorate different classes of objects" do