[ruby/stringio] StringIO#initialize default to the source string encoding
[ruby/stringio] StringIO#initialize default to the source string encoding

[Bug #16497]

https://github.com/ruby/stringio/commit/4958a5ccab

nobu/rubyspec,eregon/rubyspec,ruby/spec,eregon/rubyspec,nobu/rubyspec,ruby/spec,eregon/rubyspec,ruby/spec,nobu/rubyspec