Fix async generators finalization to work correctly in debug mode

Closes #200.

Author: 1st1

Diff for: uvloop/loop.pyx

@@ -2913,10 +2913,7 @@ cdef class Loop:
     def _asyncgen_finalizer_hook(self, agen):
         self._asyncgens.discard(agen)
         if not self.is_closed():
-            self.create_task(agen.aclose())
-            # Wake up the loop if the finalizer was called from
-            # a different thread.
-            self.handler_async.send()
+            self.call_soon_threadsafe(self.create_task, agen.aclose())
 
     def _asyncgen_firstiter_hook(self, agen):
         if self._asyncgens_shutdown_called: