Adjust handling of nil parameter for permitted hash.

When a nil param value arrives in place of an expected hash, return :key => nil, not simply nil.

Prior:

```ruby
> params
=> {"book"=>nil}
> params.permit :book => { :genre => :type }
=> {}
```

Now:

```ruby
> params
=> {"book"=>nil}
> params.permit :book => { :genre => :type }
=> {"book"=>nil}
```

Note: This is a redo of https://github.com/rails/strong_parameters/pull/20. I will close that PR.