Hash#[]= will not duplicate already frozen string key.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

key = "foo".freeze
h = {}
h[key] = 42

assert_equal( h.keys[0].object_id, key.object_id )

puts :ok
}}}

git-svn-id: http://svn.macosforge.org/repository/ruby/MacRuby/trunk@5295 23306eb0-4c56-4727-a40e-e92c0eb68959

hash.c

@@ -964,7 +964,7 @@ VALUE
 rhash_aset(VALUE hash, SEL sel, VALUE key, VALUE val)
 {
     rhash_modify(hash);
-    if (TYPE(key) == T_STRING) {
+    if (TYPE(key) == T_STRING && !OBJ_FROZEN(key)) {
         key = rb_str_dup(key);
         OBJ_FREEZE(key);
     }