Only Draper::Decorator types should delegate === to object. (#720)

Draper::Decorater provides the object method and we should delegate === to this
method only for these types.

Other classes could respond_to?(:object) so using is_a? clarifies not just any
class implementing object but only classes that are a Draper::Decorator.

Besides this, another major reason is is_a? is much faster than respond_to?
in ActiveRecord models because of the number of methods AR provides and the
layers of delegation. There is a cache specifically for respond_to? [1] because
it's so slow.

Even with this cache, some of our pages are 10% faster by using the is_a? change
from this patch.

The original implementation [2] used the method source, which was later renamed
to object.  The tests seem to indicate the intention was to always have
this === delegation to source/object for decorated objects... which is what
Drapper::Decorator types are.

Test changes:
* verify decorated instances of Product and instances derived from
decorated instances of Product are === Product.
* verify unrelated decorated instances and non-decorators that
happen to implement object are not === Product.

[1] https://github.com/rails/rails/blob/4-2-stable/activemodel/lib/active_model/attribute_methods.rb#L343-L351
[2] ca116154f3

lib/draper/decoratable.rb

@@ -86,7 +86,7 @@ module Draper
       #
       # @return [Boolean]
       def ===(other)
-        super || (other.respond_to?(:object) && super(other.object))
+        super || (other.is_a?(Draper::Decorator) && super(other.object))
       end
 
     end

spec/draper/decoratable_spec.rb

@@ -109,19 +109,25 @@ module Draper
       end
 
       it "is true for a decorated instance" do
-        decorator = double(object: Product.new)
+        decorator = Product.new.decorate
 
         expect(Product === decorator).to be_truthy
       end
 
       it "is true for a decorated derived instance" do
-        decorator = double(object: Class.new(Product).new)
+        decorator = Class.new(Product).new.decorate
 
         expect(Product === decorator).to be_truthy
       end
 
       it "is false for a decorated unrelated instance" do
-        decorator = double(object: Model.new)
+        decorator = Other.new.decorate
+
+        expect(Product === decorator).to be_falsey
+      end
+
+      it "is false for a non-decorator which happens to respond to object" do
+        decorator = double(object: Product.new)
 
         expect(Product === decorator).to be_falsey
       end