Fix html_escape failure with frozen strings

Closes #652

lib/haml/helpers.rb

@@ -587,14 +587,12 @@ MESSAGE
     if RUBY_VERSION >= '1.9'
       def html_escape(text)
         text = text.to_s
-        text.gsub!(HTML_ESCAPE_REGEX, HTML_ESCAPE)
-        text
+        text.gsub(HTML_ESCAPE_REGEX, HTML_ESCAPE)
       end
     else
       def html_escape(text)
         text = text.to_s
-        text.gsub!(HTML_ESCAPE_REGEX) {|s| HTML_ESCAPE[s]}
-        text
+        text.gsub(HTML_ESCAPE_REGEX) {|s| HTML_ESCAPE[s]}
       end
     end
 

test/helper_test.rb

@@ -497,6 +497,14 @@ HAML
     assert_equal "&quot;&gt;&lt;&amp;", Haml::Helpers.html_escape('"><&')
   end
 
+  def test_html_escape_should_work_on_frozen_strings
+    begin
+      assert Haml::Helpers.html_escape('foo'.freeze)
+    rescue => e
+      flunk e.message
+    end
+  end
+
   def test_html_escape_encoding
     old_stderr, $stderr = $stderr, StringIO.new
     string = "\"><&\u00e9" # if you're curious, u00e9 is "LATIN SMALL LETTER E WITH ACUTE"