set up config_accessor with a default value by block

* ActiveSupport::Configurable should allow config_accessor to take
  default value by block, just like cattr_accessor.

    class User
      include ActiveSupport::Configurable
      config_accessor :hair_colors do
        [:brown, :black, :blonde, :red]
      end
    end

    User.hair_colors # => [:brown, :black, :blonde, :red]

* remove trailing whitespaces in configurable.rb and its test file.

* Update ActiveSupport CHANGELOG.

activesupport/CHANGELOG.md

@@ -1,5 +1,18 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   An optional block can be passed to `config_accessor` to set its default value
+
+        class User
+          include ActiveSupport::Configurable
+          config_accessor :hair_colors do
+            [:brown, :black, :blonde, :red]
+          end
+        end
+
+        User.hair_colors # => [:brown, :black, :blonde, :red]
+
+    *Larry Lv*
+
 *   ActiveSupport::Benchmarkable#silence has been deprecated due to its lack of
     thread safety. It will be removed without replacement in Rails 4.1. *Steve
     Klabnik*

activesupport/lib/active_support/configurable.rb

@@ -92,6 +92,17 @@ module ActiveSupport
       #
       #   User.new.allowed_access = true # => NoMethodError
       #   User.new.allowed_access        # => NoMethodError
+      #
+      # Also you can pass a block to set up the attribute with a default value.
+      #
+      #   class User
+      #     include ActiveSupport::Configurable
+      #     config_accessor :hair_colors do
+      #       [:brown, :black, :blonde, :red]
+      #     end
+      #   end
+      #
+      #   User.hair_colors # => [:brown, :black, :blonde, :red]
       def config_accessor(*names)
         options = names.extract_options!
 
@@ -108,6 +119,7 @@ module ActiveSupport
             class_eval reader, __FILE__, reader_line unless options[:instance_reader] == false
             class_eval writer, __FILE__, writer_line unless options[:instance_writer] == false
           end
+          send("#{name}=", yield) if block_given?
         end
       end
     end

activesupport/test/configurable_test.rb

@@ -48,6 +48,18 @@ class ConfigurableActiveSupport < ActiveSupport::TestCase
     assert !instance.respond_to?(:baz=)
   end
 
+  test "configuration accessors can take a default value" do
+    parent = Class.new do
+      include ActiveSupport::Configurable
+      config_accessor :hair_colors, :tshirt_colors do
+        [:black, :blue, :white]
+      end
+    end
+
+    assert_equal [:black, :blue, :white], parent.hair_colors
+    assert_equal [:black, :blue, :white], parent.tshirt_colors
+  end
+
   test "configuration hash is available on instance" do
     instance = Parent.new
     assert_equal :bar, instance.config.foo