[Sass] Fix a bug in css2sass exposed by css2sass_test.

lib/sass/css.rb

@@ -377,7 +377,7 @@ module Sass
     def fold_commas(root)
       prev_rule = nil
       root.children.map! do |child|
-        next child unless Tree::RuleNode === child
+        next child unless child.is_a?(Tree::RuleNode)
 
         if prev_rule && prev_rule.children == child.children
           prev_rule.rule << ", #{child.rule}"

lib/sass/tree/attr_node.rb

@@ -9,6 +9,10 @@ module Sass::Tree
       super(value, style)
     end
 
+    def ==(other)
+      self.class == other.class && name == other.name && super
+    end
+
     def to_s(tabs, parent_name = nil)
       if value[-1] == ?;
         raise Sass::SyntaxError.new("Invalid attribute: #{declaration.dump} (This isn't CSS!).", @line)

lib/sass/tree/node.rb

@@ -17,6 +17,10 @@ module Sass
         @children << child
       end
 
+      def ==(other)
+        self.class == other.class && other.children == children
+      end
+
       def to_s
         result = String.new
         children.each do |child|

lib/sass/tree/rule_node.rb

@@ -9,6 +9,10 @@ module Sass::Tree
     alias_method :rule, :value
     alias_method :rule=, :value=
 
+    def ==(other)
+      self.class == other.class && rules == other.rules && super
+    end
+
     def rules
       Array(rule)
     end

lib/sass/tree/value_node.rb

@@ -9,6 +9,10 @@ module Sass::Tree
       super(style)
     end
 
+    def ==(other)
+      self.class == other.class && value == other.value && super
+    end
+
     def to_s(tabs = 0)
       value
     end