making it so that :parent and :class can coexist in a factory definition

Signed-off-by: Nick Quaranto <nick@quaran.to>

lib/factory_girl/factory.rb

@@ -70,7 +70,7 @@ def initialize (name, options = {}) #:nodoc:
   end
 
   def inherit_from(parent) #:nodoc:
-    @options[:class] = parent.class_name
+    @options[:class] ||= parent.class_name
     parent.attributes.each do |attribute|
       unless attribute_defined?(attribute.name)
         @attributes << attribute.clone