Implements the StringIO#set_encoding.

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

str = "foo\nbar\n"
s = StringIO.new(str, "r")
t = s.set_encoding("UTF-8")
assert_equal(StringIO, t.class)
assert_equal("UTF-8", s.string.encoding.to_s)

str.freeze
s.set_encoding("euc-jp")
assert_equal("EUC-JP", s.string.encoding.to_s)

puts :ok
}}}

lib/stringio.rb

@@ -678,6 +678,11 @@ def internal_encoding
     nil
   end
 
+  def set_encoding(enc)
+    @string = @string.encode(enc)
+    self
+  end
+
   protected
 
     # meant to be overwritten by developers