Set internal encoding instead of external

    rb:utf-8

Sets the external encoding
http://ruby-doc.org/core-1.9.3/IO.html#method-c-new-label-Mode

This will cause problems with any files using a different encoding set by the
magic comment
http://ruby-doc.org/core-2.0.0/Encoding.html#class-Encoding-label-Script+encoding

Example
http://github.com/rubygems/rubygems/blob/95f6760/lib/rubygems/package/tar_input.rb

This was predicted by @jayferd in d74facd