Don't output transparent in css2 ouput.

Closes GH-309

lib/compass/sass_extensions/monkey_patches/browser_support.rb

@@ -92,4 +92,22 @@ module Sass::Script
     end
   end
 
+  class String < Literal
+    def supports?(aspect)
+      aspect == "css2" && value == "transparent" && type == :identifier 
+    end
+
+    def has_aspect?
+      true
+    end
+
+    def to_css2(options = self.options)
+      if value == "transparent" && type == :identifier
+        Sass::Script::String.new("")
+      else
+        dup
+      end
+    end
+  end
+
 end

test/sass_extensions_test.rb

@@ -87,6 +87,7 @@ class SassExtensionsTest < Test::Unit::TestCase
     assert_equal "true", evaluate("blank('')")
     assert_equal "true", evaluate("blank(' ')")
     assert_equal "true", evaluate("blank(-compass-space-list(' '))")
+    assert_equal " ", evaluate("-css2(transparent linear-gradient(#fff,#000))")
   end
 
 protected