Bug#27127, Incorrect behaviour of timestamp column with DEFAULT CURRENT_TIMESTAMP

correct the bitmap_set_bit when a field is timestamp and described 
with default CURRENT_TIMESTAMP or on update CURRENT_TIMESTAMP,
then it will reduce a little time cost when the field doesnot need 
to write.

sql/sql_update.cc

@@ -201,6 +201,8 @@ int mysql_update(THD *thd,
       table->timestamp_field_type= TIMESTAMP_NO_AUTO_SET;
     else
     {
+      if (table->timestamp_field_type == TIMESTAMP_AUTO_SET_ON_UPDATE ||
+          table->timestamp_field_type == TIMESTAMP_AUTO_SET_ON_BOTH)
         bitmap_set_bit(table->write_set,
                        table->timestamp_field->field_index);
     }