``alias`` make decoding buggy

Hi folks,

I noticed the following behaviour

```
> base32.decode(base32.encode("deadbeef111"))
< "d{adbe{f111"
```

which appers to be introduced by the `alias` behaviour. I nulled the `aliases` dictionnary to retablish the desired behaviour.

```
> base32.decode(base32.encode("deadbeef111"))
< "deadbeef111"
```
