Apply mabotelh's fix for filters when using Mongoid

https://github.com/mabotelh/activeadmin-mongoid/commit/aeb2687dc636a019787fc080ee8964bba641cfa3

https://github.com/elia/activeadmin-mongoid/issues/38
Merge pull request #1 from monfresh/filter-fix

Apply mabotelh's fix for filters when using Mongoid