Convert rs to match stringio encoding

[nobu]

Fix #78

[kou]

The current behavior is compatible with IO:

str = "\xFE\xFFColumn1,Column2\n" + "\"value1.1\",value1.2\n"
File.write("/tmp/xxx.txt", str)
File.open("/tmp/xxx.txt", "rb", encoding: "utf-16") do |io|
  io.gets *["\n", 8192]
end

If we want to change the behavior, we may want to change the IO's behavior too.

[kou]

FYI: https://github.com/ruby/ruby/blob/42168802618381b7b6308acc290aabdf61c10116/io.c#L4085-L4087

[jherdman]

Just wondering what's happening here. We're stuck on stringio 3.0.8.

[kou]

IO has a similar problem: https://bugs.ruby-lang.org/issues/20819

[kou]

@nobu Could you resolve conflict?