Named parameters with types have types overridden by default value in hover

[jakebailey]

from typing import Optional

def opt_none(x: Optional[int] = None):
    return x

This doesn't impact the actual analysis, though.

Replaces #377.

[MikhailArkhipov]

Fixed in #680

[jakebailey]

I'm not sure if that's really the right thing to do; shouldn't it remain Optional[int]? Or are we intending to drop Optional in places?