IO: eliminate unnecessary resume at close+flush

we can do that because close do better resume suspensions later

Author: Sergey Mashkov

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

@@ -69,7 +69,7 @@ internal class ByteBufferChannel(
         if (closed != null) return false
         val newClosed = if (cause == null) ClosedElement.EmptyCause else ClosedElement(cause)
         if (!Closed.compareAndSet(this, null, newClosed)) return false
-        flush()
+        state.capacity.flush()
         if (state.capacity.isEmpty() || cause != null) tryTerminate()
         resumeClosed(cause)
         return true