Coerce on List elements produces unexpect results

Slight modification of the example at: http://cerberus.readthedocs.org/en/latest/usage.html

```
schema = {'a_list': {'type': 'list', 'schema': {'type': 'float', 'coerce': float}}}
document = {'a_list': [3, 4, 5]}
v.validated(document, schema)
>>> {0: 3.0, 1: 4.0, 2: 5.0, 'a_list': [3, 4, 5]}
```

I would expect:

```
>>> {'a_list': [3.0, 4.0, 5.0]}
```

Is this the intended behavior or a bug?
