Flush file buffers on CloseWrite

This forces any pending data to be sent to the remote party before
emitting the 0-length EOF message, to make sure it appears as a seperate
message.

This fixes the "TestEchoWithMessaging" test

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

file.go

@@ -219,3 +219,7 @@ func (f *win32File) SetWriteDeadline(t time.Time) error {
 	f.writeDeadline = t
 	return nil
 }
+
+func (f *win32File) Flush() error {
+	return syscall.FlushFileBuffers(f.handle)
+}