assign_attributes should return if argument is blank.

If you are passed an empty hash, then assign_attributes doesn't need to
do any work and can just return early. This should fix the failing Agile
Web Development tests.

activerecord/lib/active_record/attribute_assignment.rb

@@ -15,6 +15,7 @@ def assign_attributes(new_attributes)
       if !new_attributes.respond_to?(:stringify_keys)
         raise ArgumentError, "When assigning attributes, you must pass a hash as an argument."
       end
+      return if new_attributes.blank?
 
       attributes                  = new_attributes.stringify_keys
       multi_parameter_attributes  = []