Only set EAGAIN when written == 0 and length > 0. Fixes #2957

Because we use NIO channels for all IO, but need to behave like
POSIX IO functions, we emulate errno behavior for various cases.
This particular issue is an edge case, when writing zero bytes to
a stream marked as nonblocking (as happens after it is used in an
IO.select operation); we see that it's nonblocking and nothing was
written and unconditionally treat that as EAGAIN. Instead, we
should only set EAGAIN when zero bytes are written and we actually
did have bytes to write.

Author: headius

core/src/main/java/org/jruby/util/io/PosixShim.java

@@ -91,7 +91,7 @@ public int write(ChannelFD fd, byte[] bytes, int offset, int length, boolean non
 
             int written = fd.chWrite.write(tmp);
 
-            if (written == 0) {
+            if (written == 0 && length > 0) {
                 // if it's a nonblocking write against a file and we've hit EOF, do EAGAIN
                 if (nonblock) {
                     errno = Errno.EAGAIN;