Fixes bug in mongoid_adapter with empty conditions hash

* adds mongoid query that matches every record when
rule.conditions.empty? is true

lib/cancan/model_adapters/mongoid_adapter.rb

@@ -30,7 +30,7 @@ module CanCan
         else
           @rules.inject(@model_class.all) do |records, rule|
             if rule.conditions.empty?
-              records
+              records.or(:_id => {'$exists' => true}) # match everything in Mongoid
             elsif rule.base_behavior
               records.or(rule.conditions)
             else

spec/cancan/model_adapters/mongoid_adapter_spec.rb

@@ -68,6 +68,15 @@ if ENV["MODEL_ADAPTER"] == "mongoid"
         MongoidProject.accessible_by(@ability, :read).entries.should == [sir]
       end
 
+      it "should be able to mix empty conditions and hashes" do
+        @ability.can :read, MongoidProject
+        @ability.can :read, MongoidProject, :title => 'Sir'
+        sir  = MongoidProject.create(:title => 'Sir')
+        lord = MongoidProject.create(:title => 'Lord')
+
+        MongoidProject.accessible_by(@ability, :read).count.should == 2
+      end
+
       it "should return everything when the defined ability is manage all" do
         @ability.can :manage, :all
         sir   = MongoidProject.create(:title => 'Sir')