Fix completion params

I ran into the following problem when trying to grab the params from a completion:
````python
>>> import jedi
>>> c = jedi.Script('import string; string.capwords').completions()[0]
>>> c.params # raises AttributeError
>>> c._definition.params # but this works
[<Param: s @33,13>, <Param: sep = None @33,16>]
````
To me, it looked like the `pr` to `er` cast should always happen irregardless of `self._definition` being a `pr.Name`. But I'm not too confident here. :)
Cheers