Environment

$ jruby -v
jruby 9.1.17.0 (2.3.3) 2018-04-20 d8b1ff9 Java HotSpot(TM) 64-Bit Server VM 25.40-b25 on 1.8.0_40-b27 +jit [darwin-x86_64]
$ uname -a
Darwin Jankos-MacBook-Pro-2.local 17.5.0 Darwin Kernel Version 17.5.0: Mon Mar  5 22:24:32 PST 2018; root:xnu-4570.51.1~1/RELEASE_X86_64 x86_64

Expected Behavior

IO.copy_stream on MRI accepts any object that responds to either #read or #readpartial as the source, so the following script works:

require "stringio"

class FakeIO
  def initialize(content)
    @io = StringIO.new(content)
  end

  def readpartial(*args)
    @io.readpartial(*args)
  end
end

IO.copy_stream(FakeIO.new("content"), File::NULL)

Actual Behavior

JRuby raises an exception when executing the above script:

ArgumentError: Should be String or IO
  copy_stream at org/jruby/RubyIO.java:4222
       <main> at script.rb:13

If I add a FakeIO#read method, the script executes without exceptions, and it calls FakeIO#read (MRI and Rubinius choose FakeIO#readpartial if both #read and #readpartial are available). It would be nice if JRuby supported #readpartial as well, it's needed for this PR.