IO: eliminate race in close

I knew that I'll hit this bug anyway. Here we MUST flush twice as
flush at read suspend is not enough. If a writer closes then the first
flush does flush and it closes so all readers will see all pending data.
If a foreign thread does close then the first flush is makes no
difference however the second is required for proper cleanup and
to do tryTerminate

Author: Sergey Mashkov

core/kotlinx-coroutines-io/src/main/kotlin/kotlinx/coroutines/experimental/io/ByteBufferChannel.kt

@@ -71,6 +71,7 @@ internal class ByteBufferChannel(
     override fun close(cause: Throwable?): Boolean {
         if (closed != null) return false
         val newClosed = if (cause == null) ClosedElement.EmptyCause else ClosedElement(cause)
+        state.capacity.flush()
         if (!Closed.compareAndSet(this, null, newClosed)) return false
         state.capacity.flush()
         if (state.capacity.isEmpty() || cause != null) tryTerminate()