allow deletion of many_to_one target via generated writer
allow deletion of many_to_one target via generated writer

* this makes all belongs_to_specs pass, however it doesn't
  work for has_1_specs.

* before this changeset, both belongs_to and has_1 failed
  at the same spec (deleting the related resource). i wonder
  why that is ...
snusnu/dm-accepts_nested_attributes