assign_attributes? on Strong Parameters should use params

The standard on strong_parameters is to use the follow syntax to check
the params:
```ruby
expose(:post, attributes: :post_params)

def post_params
  params.require(:post)
end
```
However if we touch the attributes without having a full post on
parameters we would get a `ActionController::ParameterMissing`. In order
to fix that, method `assign_attributes?` on strong parameter strategy
should check against the params instead of attributes.

[fixes #99]