Do not delegate to Kernel methods.

This patch prevents Entity from delegating attributes
to Kernel methods. This is accomplished by testing the
methods the Entity class responds to for ownership
by the Entity class itself.

Fixes #215.

Author: Malte Rohde

lib/grape_entity/entity.rb

@@ -478,7 +478,7 @@ def value_for(key, options = Options.new)
     end
 
     def delegate_attribute(attribute)
-      if respond_to?(attribute, true)
+      if respond_to?(attribute, true) && method(attribute).owner == self.class
         send(attribute)
       else
         delegator.delegate(attribute)

spec/grape_entity/entity_spec.rb

@@ -1548,6 +1548,18 @@ def name
         expect(rep.value_for(:name)).to eq 'cooler name'
       end
 
+      it 'does not delegate to Kernel methods' do
+        module EntitySpec
+          class DelegatingEntity < Grape::Entity
+            expose :system
+          end
+        end
+
+        foo = double 'Foo', system: 'System'
+        rep = EntitySpec::DelegatingEntity.new foo
+        expect(rep.value_for(:system)).to eq 'System'
+      end
+
       context 'using' do
         before do
           module EntitySpec