Revert "Revert "Assert primary key does not exist in habtm when the association is defined, instead of doing that everytime a record is inserted.""

This reverts commit 2b82708b.

[#3128 state:resolved]

Conflicts:

	activerecord/lib/active_record/associations.rb
	activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb