fix filter for mongoid (Fix #60)

lib/impressionist/models/mongoid/impressionist/impressionable.rb

@@ -37,7 +37,8 @@ module Impressionist
     def impressionist_count(options={})
       options.reverse_merge!(:filter=>:request_hash, :start_date=>nil, :end_date=>Time.now)
       imps = options[:start_date].blank? ? impressions : impressions.between(created_at: options[:start_date]..options[:end_date])
-      options[:filter] == :all ? imps.count : imps.where(options[:filter].ne => nil).count
+      filter = options[:filter]
+      filter == :all ? imps.count : imps.where(filter.ne => nil).distinct(filter).count
     end
 
     def update_impressionist_counter_cache