foreign key in has_many broken

for example:

```ruby
belongs_to :duplicate_of, class_name: 'Report', required: false
has_many :duplicates, class_name: 'Report', foreign_key: 'duplicate_of_id'
```

breaks because of this code in the `ClassMethods#_react_param_conversion` method:

```ruby
assoc = associations.detect do |poly_assoc|
if key == poly_assoc.polymorphic_type_attribute
model_name = value
already_processed_keys << poly_assoc.association_foreign_key
elsif key == poly_assoc.association_foreign_key
model_id = value
already_processed_keys << poly_assoc.polymorphic_type_attribute
end
end
```

it is treating every relationship with a foreign key as a polymorphic relationship.

Clearly a regression when polymorphic was added