asyncio: sync with Tulip

- Tulip issue 185: Add a create_task() method to event loops. The create_task()
  method can be overriden in custom event loop to implement their own task
  class. For example, greenio and Pulsar projects use their own task class. The
  create_task() method is now preferred over creating directly task using the
  Task class.
- tests: fix a warning
- fix typo in the name of a test function
- Update AbstractEventLoop: add new event loop methods; update also the unit test

Lib/asyncio/tasks.py

@@ -505,7 +505,9 @@ def async(coro_or_future, *, loop=None):
             raise ValueError('loop argument must agree with Future')
         return coro_or_future
     elif coroutines.iscoroutine(coro_or_future):
-        task = Task(coro_or_future, loop=loop)
+        if loop is None:
+            loop = events.get_event_loop()
+        task = loop.create_task(coro_or_future)
         if task._source_traceback:
             del task._source_traceback[-1]
         return task