Concat with async iterables

I'm trying to adapt the concat operator to implement the catch_exception op, but just noticed that the concat fails with an async iterable. I'm using the following observable:

```
async def asynciter():
for i in range(5):
await asyncio.sleep(1)
yield i
xs = from_async_iterable(asynciter())
```

I have a fork with a working catch_exception and retry here, as well as the tests mentioned: https://github.com/tr11/aioreactive/commit/6219f9a0761f69fa1765129b990762affdf661c8

Start with the concat operator and the from_async_iterable example in the issue. Compare the working catch_exception and retry changes and their tests in commit 6219f9a0761f69fa1765129b990762affdf661c. Done means concat works with the shown async iterable and the mentioned tests pass.

Written by the indexing model from the issue text.