Make Blank object serializable with Marshal

[Fix #242]

lib/jbuilder.rb

@@ -1,4 +1,5 @@
 require 'jbuilder/jbuilder'
+require 'jbuilder/blank'
 require 'jbuilder/key_formatter'
 require 'jbuilder/errors'
 require 'multi_json'
@@ -21,7 +22,7 @@ def self.encode(*args, &block)
     new(*args, &block).target!
   end
 
-  BLANK = ::Object.new
+  BLANK = Blank.new
 
   def set!(key, value = BLANK, *args, &block)
     result = if block

lib/jbuilder/blank.rb

@@ -0,0 +1,7 @@
+class Jbuilder
+  class Blank
+    def ==(other)
+      super || Blank === other
+    end
+  end
+end

test/jbuilder_template_test.rb

@@ -190,6 +190,27 @@ def assert_collection_rendered(json, context = nil)
     assert_equal '{"posts":[]}', json
   end
 
+  test 'cache an empty block' do
+    undef_context_methods :fragment_name_with_digest, :cache_fragment_name
+
+    render_jbuilder <<-JBUILDER
+      json.cache! 'nothing' do
+      end
+    JBUILDER
+
+    json = nil
+
+    assert_nothing_raised do
+      json = render_jbuilder <<-JBUILDER
+        json.foo 'bar'
+        json.cache! 'nothing' do
+        end
+      JBUILDER
+    end
+
+    assert_equal 'bar', MultiJson.load(json)['foo']
+  end
+
   test 'fragment caching a JSON object' do
     undef_context_methods :fragment_name_with_digest, :cache_fragment_name