Add tests for incorrect transaction handling

[tailhook]

This is only a test. I think you'll come up with a fix pretty easily.

In case it's not obvious value in line 247 is QUEUED

[popravich]

Thanks,
I wil add assertion in pool.release method

[tailhook]

I believe assertion is not good or not enough, because it may hide real exception. It's better to close multi-exec or close connection. I.e. in case like:

try:
  with (yield from pool) as redis:
    redis.multi()
    1/0
    redis.exec()
except ZeroDivisionError:
   print('zero division')

Would be nice to see exception and be able to catch it instead of getting assertion error

[popravich]

that makes sense