(Merge 3.4) asyncio: BaseEventLoop._assert_is_current_event_loop() now only

(Merge 3.4) asyncio: BaseEventLoop._assert_is_current_event_loop() now only
raises an exception if the current loop is not None.

Guido van Rossum wrote:

"The behavior that you can set the loop to None (and keep track of it
explicitly) is part of the spec, and this should still be supported even in
debug mode. The behavior that we raise an error if you are caught having
multiple active loops per thread is just a debugging heuristic, and it
shouldn't break code that follows the spec."