ActiveRecord::Base#self_and_descendents_from_active_record is misspelled

It should be descendants, with an A. In standard usage, "descendant" is the noun and "descendent" is the adjective. This is also the common usage across the Rails codebase.

Normally I don't know if I'd change an established method name just for that, but 1) it's marked :nodoc: and 2) there's another method in AR::Base called #class_of_active_record_descendant, and it's weird that the two aren't consistent.

Here's a patch!