Fix #136 - Allow bDestroy / bRetrieve to have defaults set

- Typically I think setting bDestroy or bRetrieve as default true is a
  bad idea as it could lead to more processing of tables than is needed
  by mistake, but if set the defaults should be acted upon.

- These are slightly different to the other defaults since the settings
  object hasn't been expanded by the point they are checked, so need to
  manually check the values.