Fix example fiber scheduler reg. writable events

There were two issues:

1. When an IO object is waiting for writablility only (as in test_tcp_accept) the selected hash is empty.
   Therefore selected[fiber] returns nil but needs to default to 0 in order to be or'ed with IO::WRITABLE.

2. When an IO object is waiting for read- or writability (as in test_tcp_connect), but only one of these
   two events arrive, the Fiber and IO object need to be removed from the other `@readable` or `@writable` list.

test/fiber/scheduler.rb

@@ -60,6 +60,7 @@ class Scheduler
 
       readable&.each do |io|
         if fiber = @readable.delete(io)
+          @writable.delete(io) if @writable[io] == fiber
           selected[fiber] = IO::READABLE
         elsif io == @urgent.first
           @urgent.first.read_nonblock(1024)
@@ -68,7 +69,8 @@ class Scheduler
 
       writable&.each do |io|
         if fiber = @writable.delete(io)
-          selected[fiber] |= IO::WRITABLE
+          @readable.delete(io) if @readable[io] == fiber
+          selected[fiber] = selected.fetch(fiber, 0) | IO::WRITABLE
         end
       end
 

test/fiber/test_io.rb

@@ -97,4 +97,47 @@ class TestFiberIO < Test::Unit::TestCase
 
     assert_kind_of Errno::EPIPE, error
   end
+
+  def test_tcp_accept
+    server = TCPServer.new('localhost', 0)
+
+    th = Thread.new do
+      Fiber.set_scheduler(Scheduler.new)
+
+      Fiber.schedule do
+        sender = server.accept
+        sender.wait_writable
+        sender.write "hello"
+        sender.close
+      end
+    end
+
+    recver = TCPSocket.new('localhost', server.local_address.ip_port)
+    assert "hello", recver.read
+
+    recver.close
+    server.close
+    th.join
+  end
+
+  def test_tcp_connect
+    server = TCPServer.new('localhost', 0)
+
+    th = Thread.new do
+      Fiber.set_scheduler(Scheduler.new)
+
+      Fiber.schedule do
+        sender = TCPSocket.new('localhost', server.local_address.ip_port)
+        sender.write "hello"
+        sender.close
+      end
+    end
+
+    recver = server.accept
+    assert "hello", recver.read
+
+    recver.close
+    server.close
+    th.join
+  end
 end