Remove obsolete code

Update RDoc to mention integer coercion in ActiveRecord’s find by id

The main reason for this is that I want to separate the code that does
the mutating from the code that does the cloning.

#send_file leans on Rack::Sendfile to X-Accel-Redirect the file's path, so opening the file to set the response body is wasteful. Set a FileBody wrapper instead that responds to to_path and streams the file if needed.

Improve the performance of #delegate

Some refactor for association.

* Remove unused association_class method.
* Remove a unnecessary assignment.
* Move @updated to BelongsToAssociation that only reference this instance variable.
* Reset @stale_state at the reset method. I think this place is right place.

Remove the use of #__send__ in order to boost performance. This also
means that you can no longer delegate to private methods on the
target object.

Add method FormBuilder#index

Add index method to FormBuilder. Useful when you use field_for and need to know the index number into the iteration.

Add better errors reporting for ActiveModel::Configuration

Get a properly aliased_table_name, when we use a polymorphic association.

Override AR::Relation methods in NullRelation.

Could also be worded 'as an option', but plural made more sense given 
the mention of ':handlers and :formats'.

This reverts commit f28f1d70.

Reason: This is already covered in this guide elsewhere.