Allow specifying a field to identify new records.

It is possible to use 'slugged' resource URLs using the `primary_key` method, however doing so causes her to consider the presence or absence of the 'slug' field as an indicator of whether the record is `new?`

This PR makes it possible to specify a different field to identify whether a record is new or not, which defaults to the same field as `primary_key`

    class MySluggedResource
      primary_key 'slug'
      new_record_key 'id'
    end