Fix the encoding of the backend string with StringIO.new

* #set_encoding would check @writable which is not set yet in the constructor.

Author: eregon

lib/truffle/stringio.rb

@@ -69,8 +69,7 @@ def self.open(*args)
 
   def initialize(string=nil, mode=nil)
     if string.nil?
-      @__data__ = Data.new ''
-      set_encoding(nil)
+      @__data__ = Data.new ''.force_encoding(Encoding.default_external)
       mode = IO::RDWR
     else
       string = Truffle::Type.coerce_to string, String, :to_str