Fix issues in query_spec

* `published_between` scope defined on `PostQuery::Repository` was not
  calling `where` on `relation` property. Error was being swallowed by
  `ActiveRecord::Relation#scoping`
* Change test to verify scopes are skipped if query params are empty by
  creating a record that should be filtered out if scopes are not
  skipped.
Enable setting defaults in param processors

If `unfiltered_params` has no entries for any of the `Query` object's
keys, then the call to `inject` will iterate over an empty hash. Using
`keys` ensures that any param processors defined on `ParamsFilter` get
called even if there isn't an original value.