Implement #== for column

We shouldn't rely on reference equality of these objects in tests

activerecord/lib/active_record/connection_adapters/column.rb

@@ -56,6 +56,14 @@ def with_type(type)
           clone.instance_variable_set('@cast_type', type)
         end
       end
+
+      def ==(other)
+        other.name == name &&
+          other.default == default &&
+          other.cast_type == cast_type &&
+          other.sql_type == sql_type &&
+          other.null == null
+      end
     end
   end
   # :startdoc: