Register subclasses of subclasses of Jekyll::Plugin

The Sass and SCSS converters are practically the same – only different in
the input syntax and file extension. As such, we've created
`Jekyll::Converters::Scss` which is a subclass of `Jekyll::Converter`, and
`Jekyll::Converters::Sass` which is a subclass of
`Jekyll::Converters::Scss`. When `Site#instantiate_classes` is called on
`Jekyll::Converter`, it only instantiates the `Scss` converter, not the
`Sass` converter. This change fixes that.

Fixes #2334.

lib/jekyll/plugin.rb

@@ -6,22 +6,15 @@ module Jekyll
                    :high => 10,
                    :highest => 100 }
 
-    # Install a hook so that subclasses are recorded. This method is only
-    # ever called by Ruby itself.
+    # Fetch all the subclasses of this class and its subclasses' subclasses.
     #
-    # base - The Class subclass.
-    #
-    # Returns nothing.
-    def self.inherited(base)
-      subclasses << base
-      subclasses.sort!
-    end
-
-    # The list of Classes that have been subclassed.
-    #
-    # Returns an Array of Class objects.
-    def self.subclasses
-      @subclasses ||= []
+    # Returns an array of descendant classes.
+    def self.descendants
+      descendants = []
+      ObjectSpace.each_object(singleton_class) do |k|
+        descendants.unshift k unless k == self
+      end
+      descendants
     end
 
     # Get or set the priority of this plugin. When called without an

lib/jekyll/site.rb

@@ -365,7 +365,7 @@ module Jekyll
     #
     # Returns array of instances of subclasses of parameter
     def instantiate_subclasses(klass)
-      klass.subclasses.select do |c|
+      klass.descendants.select do |c|
         !safe || c.safe
       end.sort.map do |c|
         c.new(config)

test/test_sass.rb

@@ -14,5 +14,13 @@ class TestSass < Test::Unit::TestCase
     should "import SCSS partial" do
       assert_equal ".half {\n  width: 50%; }\n", File.read(@test_css_file)
     end
+
+    should "register the SCSS converter" do
+      assert !!@site.getConverterImpl(Jekyll::Converters::Scss), "SCSS converter implementation should exist."
+    end
+
+    should "register the Sass converter" do
+      assert !!@site.getConverterImpl(Jekyll::Converters::Sass), "Sass converter implementation should exist."
+    end
   end
 end