IO#readpartial now works in JRuby

I was doing some concurrency benchmarks on various Rubys and I ran into a pretty nasty bug with IO#readpartial in JRuby. On EOF it would just return an empty string forever, instead of raising an EOFError as it should. I’m pretty surprised no one ran into this yet.

Since I thought this was pretty important I submitted a patch, they took notice, did some refactoring of their IO methods, and now it passes all of the readpartial rubyspecs. So you can expect a fully functioning IO#readpartial in JRuby 1.1.7. Good job JRuby guys!