Derive foreign key from model name in has_many associations

[volmer]

Currently Active Record derives the foreign key name for has_many associations based on the Active Record Base `name. Sometimes when using additional conventions for class names, such as with class name suffixes and prefixes, it makes sense to be able to further customize the foreign key logic to account for a different pattern.

Similar to what was done in the case of table names, as Active Record Base extends Active Model Naming, we have already a model_name at the class-level that behaves similarly to name while at the same time giving more flexibility by allowing specifying a custom name and namespacing.

This commit changes the foreign key computation in Reflections to infer its value based on the Active Record's model_name object instead of its class name. This allows customization of the foreign key pattern since a distinct instance of ActiveModel::Name can be used instead.

For example, to use post_id as foreign key for a PostRecord class in a has_many association:

 class PostRecord < ActiveRecord::Base
   has_many :comments

   class << self
     def model_name
       ActiveModel::Name.new(self, nil, "Post")
     end
   end
 end

 PostRecord.reflect_on_association(:comments).foreign_key
 # => "post_id"