Removed dead code path
Removed dead code path

* Collection (children) does not respond to new, so the true branch
  would never be executed.
* Think about removing OneToMany::Proxy#new.  #build is already supported
  and does nearly the same thing.  Plus it is inconsistent with Collection's
  API, which is the interface this should emulate.

engineyard/dm-core,datamapper/dm-core,tpitale/dm-core,alloy/dm-core,tillsc/dm-core,ar-dm/ardm-core