[Haml] Handle frozen template strings.

Closes gh-197

doc-src/HAML_CHANGELOG.md

@@ -10,6 +10,10 @@
 Support for Rails 3 versions prior to beta 4 has been removed.
 Upgrade to Rails 3.0.0.beta4 if you haven't already.
 
+### Minor Improvements
+
+* Properly process frozen strings with encoding declarations.
+
 ## 3.0.12
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/3.0.12).

lib/haml/util.rb

@@ -441,6 +441,7 @@ MSG
     # @raise [ArgumentError] if the document declares an unknown encoding
     def check_haml_encoding(str, &block)
       return check_encoding(str, &block) if ruby1_8?
+      str = str.dup if str.frozen?
 
       bom, encoding = parse_haml_magic_comment(str)
       if encoding; str.force_encoding(encoding)