Fix promotion rule spec to use association
Fix promotion rule spec to use association

* #dup will do a shallow copy of the object. That means that it will
  copy the attributes but not the associations. The way that the code is
  written works because we are building new associations. The way that
  the spec was written will not work because the
  #test_rule_test_products was an attribute on the object and therefore
  copied over to the duplicated object. This would cause the
  'association' we were trying to mimick in the spec to be the same both
  in the original and dup, therefore addiding the association to the
  same array and causing an infinite loop.

bonobos/solidus,bonobos/solidus,bonobos/solidus,bonobos/solidus