Fixed ack for tombstones in cython-stream (#39)

faust/_cython/streams.pyx

@@ -81,13 +81,14 @@ cdef class StreamIterator:
         do_ack = stream.enable_acks
 
         value = None
+        event = None
 
-        while value is None:
+        while value is None and event is None:
             await sleep(0, loop=self.loop)
             need_slow_get, channel_value = self._try_get_quick_value()
             if need_slow_get:
                 channel_value = await self.chan_slow_get()
-            value, sensor_state = self._prepare_event(channel_value)
+            event, value, sensor_state = self._prepare_event(channel_value)
 
             try:
                 for processor in self.processors:
@@ -165,10 +166,10 @@ cdef class StreamIterator:
                 stream_state = self.on_stream_event_in(
                     tp, offset, self.stream, event)
             self.stream._set_current_event(event)
-            return (event.value, stream_state)
+            return (event, event.value, stream_state)
         else:
             self.stream._set_current_event(None)
-            return channel_value, stream_state
+            return None, channel_value, stream_state
 
     cdef object _try_get_quick_value(self):
         if self.chan_is_channel: