add true option for dependent matcher and test all options

Added test to ensure that all valid `:dependent` options are permitted to fix #393. Also added a `true` option that accepts any `:dependent` option. This is useful when you're testing that all `has_many` or `has_one` relationships have a `:dependent` option set.
This will create a global method and we don't like that :) Can you put this inside of the outmost `describe` block below, at the bottom?