Deprecate ActiveSupport::JSON::Variable

Reason: ActiveSupport::JSON::Variable is not used anymore internally. It
was deprecated in 3-2-stable but we reverted all the deprecation for
point releases.

See #6536 and #6546.
Conflicts:
	activesupport/lib/active_support/json/variable.rb

activesupport/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   ActiveSupport::JSON::Variable is deprecated. Define your own #as_json and #encode_json methods
+    for custom JSON string literals. *Erich Menge*
+
 *   Add String#indent. *fxn & Ace Suares*
 
 *   Inflections can now be defined per locale. `singularize` and `pluralize` accept locale as an extra argument. *David Celis*

activesupport/lib/active_support/json/variable.rb

+require 'active_support/deprecation'
+
+module ActiveSupport
+  module JSON
+    # Deprecated: A string that returns itself as its JSON-encoded form.
+    class Variable < String
+      def initialize(*args)
+        ActiveSupport::Deprecation.warn 'ActiveSupport::JSON::Variable is deprecated and will be removed in Rails 4.0. ' \
+                                        'For your own custom JSON literals, define #as_json and #encode_json yourself.'
+        super
+      end
+
+      def as_json(options = nil) self end #:nodoc:
+      def encode_json(encoder) self end #:nodoc:
+    end
+  end
+end

activesupport/test/json/encoding_test.rb

@@ -85,6 +85,13 @@ def sorted_json(json)
     end
   end
 
+  def test_json_variable
+    assert_deprecated do
+      assert_equal ActiveSupport::JSON::Variable.new('foo'), 'foo'
+      assert_equal ActiveSupport::JSON::Variable.new('alert("foo")'), 'alert("foo")'
+    end
+  end
+
   def test_hash_encoding
     assert_equal %({\"a\":\"b\"}), ActiveSupport::JSON.encode(:a => :b)
     assert_equal %({\"a\":1}), ActiveSupport::JSON.encode('a' => 1)