```ruby
nil.respond_to?(:stringify_keys) # => false
```
```ruby
include ActiveModel::AttributeAssignment
Object.new.assign_attributes(nil) # => ArgumentError: When assigning attributes, you must pass a hash as an argument.
```
The name `ActiveModel::AttributeAssignment::UnknownAttributeError` is
too implementation specific so let's move the constant directly under
the ActiveModel namespace.
Also since this constant used to be under the ActiveRecord namespace, to
make the upgrade path easier, let's avoid raising the former constant
when we deal with this error on the Active Record side.
