make collection into an accessor, fixes respond_to in attributes and …

…should be more performant

lib/fog/core/model.rb

@@ -4,11 +4,7 @@ class Model
     extend Fog::Attributes::ClassMethods
     include Fog::Attributes::InstanceMethods
 
-    attr_accessor :connection
-
-    def collection
-      @collection
-    end
+    attr_accessor :collection, :connection
 
     def initialize(new_attributes = {})
       merge_attributes(new_attributes)
@@ -48,11 +44,5 @@ def wait_for(timeout=600, interval=1, &block)
       end
     end
 
-    private
-
-    def collection=(new_collection)
-      @collection = new_collection
-    end
-
   end
 end