Adding cache support to version 0.10.0

It's a new implementation of cache based on ActiveSupport::Cache.
The implementation abstracts the cache in Adapter class on a
private method called cached_object, this method is intended
to be used on Adapters inside serializable_hash method in order
to cache each instance of the object that will be returned by
the serializer.

Some of its features are:
- A different syntax. (no longer need the cache_key method).
- An options argument that have the same arguments of ActiveSupport::Cache::Store, plus a key option that will be the prefix of the object cache on a pattern "#{key}-#{object.id}".
- It cache the objects individually and not the whole Serializer return, re-using it in different requests (as a show and a index method for example.)

test/serializers/cache_test.rb

@@ -0,0 +1,62 @@
+require 'test_helper'
+module ActiveModel
+  class Serializer
+    class CacheTest < Minitest::Test
+      def setup
+        @post            = Post.new({ title: 'New Post', body: 'Body' })
+        @comment         = Comment.new({ id: 1, body: 'ZOMG A COMMENT' })
+        @author          = Author.new(name: 'Joao M. D. Moura')
+        @role            = Role.new(name: 'Great Author')
+        @author.posts    = [@post]
+        @author.roles    = [@role]
+        @author.bio      = nil
+        @post.comments   = [@comment]
+        @post.author     = @author
+        @comment.post    = @post
+        @comment.author  = @author
+
+        @post_serializer    = PostSerializer.new(@post)
+        @author_serializer  = AuthorSerializer.new(@author)
+        @comment_serializer = CommentSerializer.new(@comment)
+      end
+
+      def test_cache_definition
+        assert_equal(ActionController::Base.cache_store, @post_serializer.class._cache)
+        assert_equal(ActionController::Base.cache_store, @author_serializer.class._cache)
+        assert_equal(ActionController::Base.cache_store, @comment_serializer.class._cache)
+      end
+
+      def test_cache_key_definition
+        assert_equal('post', @post_serializer.class._cache_key)
+        assert_equal('writer', @author_serializer.class._cache_key)
+        assert_equal(nil, @comment_serializer.class._cache_key)
+      end
+
+      def test_cache_key_interpolation_with_updated_at
+        author = render_object_with_cache_without_cache_key(@author)
+        assert_equal(nil, ActionController::Base.cache_store.fetch(@author.cache_key))
+        assert_equal(author, ActionController::Base.cache_store.fetch("#{@author_serializer.class._cache_key}/#{@author_serializer.object.id}-#{@author_serializer.object.updated_at}").to_json)
+      end
+
+      def test_default_cache_key_fallback
+        comment = render_object_with_cache_without_cache_key(@comment)
+        assert_equal(comment, ActionController::Base.cache_store.fetch(@comment.cache_key).to_json)
+      end
+
+      def test_cache_options_definition
+        assert_equal({expires_in: 0.05}, @post_serializer.class._cache_options)
+        assert_equal(nil, @author_serializer.class._cache_options)
+        assert_equal({expires_in: 1.day}, @comment_serializer.class._cache_options)
+      end
+
+      private
+      def render_object_with_cache_without_cache_key(obj)
+        serializer_class = ActiveModel::Serializer.serializer_for(obj)
+        serializer = serializer_class.new(obj)
+        adapter = ActiveModel::Serializer.adapter.new(serializer)
+        adapter.to_json
+      end
+    end
+  end
+end
+