StringIO#write will raise an IOError if string is frozen.

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

s = ""
f = StringIO.new(s, "w")
s.freeze
assert_raise(IOError){ f.write("foo") }

puts :ok
}}}

lib/stringio.rb

@@ -493,6 +493,7 @@ def write(str)
     return 0 if str.empty?
 
     raise(IOError, "not opened for writing") unless @writable
+    raise(IOError, "not modifiable string") if @string.frozen?
 
     if @append || (pos >= string.length)
       # add padding in case it's needed