fix: Check only object's private method when preventing delegati… (#875)

https://github.com/drapergem/draper/pull/849 introduced a valid fix to prevent calling object's public method when a decorator overwrites it through a private one. There is one caveat, though. `private_methods` methods has `all` parameter set to `true` by default (https://ruby-doc.org/core-2.7.0/Object.html#method-i-private_methods)`. As a result all private methods are returned, instead of only the ones defined on the object.

This commit fixes the above issue by setting the `all` parameter to `false`.

lib/draper/automatic_delegation.rb

@@ -20,7 +20,7 @@ module Draper
 
     # @private
     def delegatable?(method)
-      return if private_methods.include?(method)
+      return if private_methods(false).include?(method)
 
       object.respond_to?(method)
     end

spec/draper/decorator_spec.rb

@@ -684,6 +684,18 @@ module Draper
             expect{ decorator.hello_world }.to raise_error NoMethodError
           end
         end
+
+        context 'when delegated method has the same name as private method defined on another object' do
+          let(:decorator_class) { Class.new(Decorator) }
+          let(:object) { Class.new { def print; end }.new }
+
+          it 'delegates the public method defined on the object' do
+            decorator = decorator_class.new(object)
+
+            # `print` private method is defined on `Object`
+            expect{ decorator.print }.not_to raise_error NoMethodError
+          end
+        end
       end
 
       context ".method_missing" do