Return nil from read_attribute if _foo is defined, but @attributes ha…

…s no 'foo' key. Fixes breakage in test_find_only_some_columns on Ruby 1.9 which was introduced in a3639be.

activerecord/lib/active_record/attribute_methods/read.rb

@@ -78,7 +78,7 @@ def define_read_method(symbol, attr_name, column)
       # "2004-12-12" in a data column is cast to a date object, like Date.new(2004, 12, 12)).
       def read_attribute(attr_name)
         if respond_to? "_#{attr_name}"
-          send "_#{attr_name}"
+          send "_#{attr_name}" if @attributes.has_key?(attr_name.to_s)
         else
           _read_attribute attr_name
         end