Return result counts from update methods

Deleting an object with a <tt>has_many :dependent => :nullify</tt> gets an error updating the counter cache. This is because Relation#update_all calls @klass.connection.update, which should return the affected count. See active_record/connection_adapters/abstract/database_statements.rb: "# Executes the update statement and returns the number of rows affected."

These methods are simply copied from the Postgres implementation, like the method they are next to.

It might be worth doing a systematic check for other problems of this sort.