Support Mongoid associations in find_model/s

Given a Post model that belongs to a User, there was previously
asymmetry in that a Given step like this would work:

  Given the following posts exist:
    | title | user        |
    | foo   | user: "bob" |

but a similar Then step would not:

  Then the following posts should exist:
    | title | user        |
    | foo   | user: "bob" |