ReadDirectoryChangesW: send on closed channel

[rjeczalik]

https://ci.appveyor.com/project/rjeczalik/notify-246/build/91

=== RUN TestRecursiveTree
panic: runtime error: send on closed channel

goroutine 25 [running]:
runtime.panic(0x5ac540, 0x6db87e)
    c:/go/src/pkg/runtime/panic.c:279 +0x11f
github.com/rjeczalik/notify.(*readdcw).send(0xc0820085c0, 0xc08203a0c0, 0x1, 0x1)
    c:/projects/src/github.com/rjeczalik/notify/watcher_readdcw.go:408 +0x11b
github.com/rjeczalik/notify.(*readdcw).loopevent(0xc0820085c0, 0x22, 0xc082054600)
    c:/projects/src/github.com/rjeczalik/notify/watcher_readdcw.go:389 +0x283
github.com/rjeczalik/notify.(*readdcw).loop(0xc0820085c0)
    c:/projects/src/github.com/rjeczalik/notify/watcher_readdcw.go:336 +0x14c
created by github.com/rjeczalik/notify.(*readdcw).lazyinit
    c:/projects/src/github.com/rjeczalik/notify/watcher_readdcw.go:307 +0x15f

The dispatch channel's lifetime has not changed, yet apparently readdcw issues a dangling send. @ppknap reassign back to me if it turns out to be broken on my side.