Fix so that Hobo can be included as a plugin to a generic Rails

project without running any generators or anything. [#508 state:resolved]

hobo/lib/hobo/model.rb

@@ -85,6 +85,7 @@ def self.all_models
         @models_loaded = true
       end
 
+      @model_names ||= Set.new
       # ...but only return the ones that registered themselves
       @model_names.map do |name|
         name.safe_constantize || (@model_names.delete name; nil)